World of Warcraft perfects the free-to-play mentality.

Free-to-play game developers should expect a 70 percent drop in initial registered users, according to former Empire: Total War designer Jan van der Crabben during a keynote presentation at this year's GDC Europe.

Player retention is key for the success of free-to-play titles, Crabben explained, adding that while most players leave free-to-play titles within the first few minutes of registering, the number of users who stick with a game by the fourth day will generally continue to play. He went on to state free-to-play games must hook players within the first three minutes, while developers should consider a 30 percent retention of its registered user base to be a success.

World of Warcraft perfects what free-to-play games should thrive for, he added. The constant reward system that World of Warcraft creates is an ideal example of how to retain a user base, due to its method of having users "kill stuff to get better to get more abilities to kill more stuff.".

Crabben is currently senior game designer at Travian Games, which develops browser-based free-to-play titles such as Imperion and Travian.