The rules have evolved over time, but the latest version says that the offseason (before training camp) can be no more than 9 weeks long, unless your team has a new coach, which lengthens the period to 11 weeks. The first 2 weeks, known as Phase One, allow for strength and conditioning work only. No coaches are allowed on the field (other than strength and conditioning coaches). Also, no helmets. And, uh, no footballs are allowed on the field, either, unless a quarterback wants to throw to an undefended receiver.