No one knows when it will happen.

No one knows when Eli Manning takes a seat and Daniel Jones gets his shot.

Not Dave Gettleman, the general manager who stuck with Manning the past two miserable seasons and then stunned everyone outside the Giants’ draft room by taking Jones with the No. 6-overall pick. Not Pat Shurmur, who was adamant he wanted to embark on his Giants head coaching tenure with Manning and equally as adamant the team needed to take Jones.

Not Manning. Not Jones. Not John Mara, the co-owner who signed off on the selection of Jones and hopes Jones as a rookie never plays, because that means Manning is playing well and the Giants are winning.

No one knows about the timing. But someone once assigned to make these decisions and who has an intimate knowledge of how the Giants operation works knows how it will go down.

“You love the guy that won for you, there’s no question,’’ Ernie Accorsi, the former Giants general manager, told The Post. “I don’t know I’d use the word sentiment; I’d use the word class. It’s got to be handled with class. I know the Giants will do it that way. When the time comes, whenever it comes.

“I always said this — the most important thing, when you work for a team, is that certificate that they put on the wall, which is the franchise. That comes first. And that’s what you have to make decisions on behalf of. It’s gonna be determined by health and how the season goes. And at some point the coach has to make a decision. I don’t know how it’s going to play out. It’s played out a million different ways. When you’re talking about legends, it’s usually the quarterback in football.”

Jones will replace Manning and get the chance to carry the franchise quarterback mantle Manning held so elegantly since 2004 and continues to hold onto heading into this season. When it happens, Jones will replace the Giants record-holder in every meaningful quarterback statistical category. Manning is not only the longest-tenured player in franchise history — no one has ever before played in a 16th season for the team — he is the Giants’ only quarterback to play in and win two Super Bowls and one of only five players in NFL history to win multiple Super Bowl MVP awards. Tom Brady won it four times, Joe Montana three times and Bart Starr, Terry Bradshaw and Manning won it twice. Starr, Montana and Bradshaw are in the Pro Football Hall of Fame, Brady will be there the moment he is eligible and Manning figures to get inducted as well.

So, Eli Manning is a Giants legend, and the next in line, Daniel Jones, faces a daunting challenge replacing him.

The one who comes after a legend faces a near-impossible task. How could anyone succeed John Wooden at UCLA? Gene Bartow tried.

Didi Gregorius moved in at shortstop for the Yankees after Derek Jeter’s brilliant run came to an end and, somewhat miraculously, the pinstripes did not fall off that fabled uniform. No one could match Don Mattingly’s popularity, but Tino Martinez held the fort at first base and eventually got a plaque in Monument Park. Mike Piazza grew up with the Dodgers, but his fame expanded exponentially with the Mets, and when it was time for him to go, Paul LoDuca had some nice moments as the new catcher, but he was not Piazza. Richard Todd came after Joe Namath, and the less said about this transition, the better.

Each replacement experiences similar but slightly different variables. Most move in after the legendary player is already off the premises. Jeter’s presence and winning were off the charts and, at the time, no one could have known how perfect the choice of Gregorius was as heir to the shortstop throne.

“The biggest challenge? Just be yourself and play the game,” Gregorius told The Post’s Ken Davidoff. “That’s the only thing you can do. You can’t worry about anything.”

It was not a completely smooth transition from the start, and Gregorius heard it from Yankees loyalists for, well, not being Jeter.

“Yeah, I know,” Gregorius said. “There’s nothing I can do. I can’t control what comes out of people’s mouths.”

The succession back in 2004 went according to plan, just not Accorsi’s plan. The Giants won five of their first seven games with Kurt Warner — signed as the veteran bridge to Manning — as the starter, and after two straight losses, Tom Coughlin was primed to pull the plug and get the rookie on the field.

“I don’t want to criticize Tom, I won’t criticize Tom, but the fact is I would have been happier if he wouldn’t have played him,’’ said Accorsi, who had such a strong conviction on Manning that he engineered a draft-day trade with the Chargers to get him.

Did he mean he would have been happier if Manning did not play the entire 2004 season?

“I would have been happier,” Accorsi said. “We were rebuilding. We had a new coach, he had the quarterback. We rebuilt that entire team from the 2000 Super Bowl, we had [Amani] Toomer and [Michael] Strahan, but other than that … it was a whole new team.”

Manning lost his first six starts and veterans such as Strahan, Toomer, Tiki Barber and Keith Hamilton were incensed that Warner was benched, as they believed they could have made the playoffs with him running the offense. With Manning, they knew it was all about the future.

“Well, I hoped Kurt Warner was going to play the whole year,” Accorsi said. “We were 5-4 when we benched Warner and started Eli against Baltimore. That was not my design.”

Manning’s first four starts were against the Falcons, Eagles, Redskins and Ravens — all top-notch defensive teams.

“And Eli got killed,” Accorsi said. “That was not the way I really wanted it to go, but [Coughlin] was the coach, he did it and Eli survived.”

Accorsi was an eyewitness to, and in some cases the instigator of, several high-profile moments when legendary players were jettisoned. As a young executive in the NFL, he recounts having “a ringside seat” when Johnny Unitas was supplanted in Baltimore. “He’d be the all-time legend,’’ Accorsi said.

Unitas was past his prime in 1973 when he was traded to the San Diego Chargers.

“It ruptured the franchise,” Accorsi said. “We lost season-ticket holders, it might have indirectly led to the eventual move [to Indianapolis]. We were never the same as a franchise.”

Marty Domres, a capable player, was Unitas’ replacement in Baltimore.

“Marty Domres was a workable quarterback, but he wasn’t going to be able to succeed Unitas, he had no chance,’’ Accorsi said. “The next year we draft Bert Jones. Well, he struggled for a couple of years. In two years, he was the star of the town, he was the matinee idol of the town. MVP, he took us to three straight divisional titles before he got hurt. The way to do it is the way Dave [Gettleman] did it. How the passing of the baton occurs, some of the time it’s out of your control.”

It could be an injury that derails Manning, but he has been invulnerable in the first 15 years of his remarkably durable career. It more likely will be a decision that must be made, and when it is, a legend will be told to move aside.

“When you have to try to orchestrate it, it’s never easy,” Accorsi said. “[Brett] Favre wasn’t happy when they drafted [Aaron] Rodgers. But that’s a great way to do it. It’s just very uncomfortable to do it when you draft a successor.”

The Giants are taking the uncomfortable route. No one said telling Eli Manning it is time to go would be easy.