মার্জার become warriors when they প্রদর্শনী the ascets of one. It depends on training progress and how responsible and mature they are. But they can't be apprentices forever, and they can't be a warrior after a moon of training, so I guess they're required to train for 4-6 অথবা 7 moons. Hope this makes it clear!
Yes it does depend on training progress, for example Firepaw was apprenticed after Sandstrorm & Dustpelt yet he was made a warrior before them. Also Medicine cat apprentices take longet to become Mediine cats.