I’m writing in an NPC that will become a wizard – how long will it take them to level up? [closed]

There’s an option in my campaign that the characters can save a man cursed to be a stag, and if they can reverse the curse he will be really grateful, but doesn’t have anything to repay them with. I want to turn him into a wizard (lvl 5+ preferably) and then have him come back into contact and give them some magical calling item so he can come and fight alongside them once to help them out and settle his debt. How long does it make sense to wait (in game time) before the NPC could be Lvl 5+? I want to keep it realistic but I also don’t want to sit on it so long they forget who he is.