It depends if you are on a Long or Short campaign. I usually turtle while playing a Short campaign and I still complete my objectives, so that just goes to show you the time given is plentiful. But you also have to take into consideration what faction you are playing and what their objectives are. For instance, it would be easier to complete a France campaign because they only need to eliminate England, as opposed to Venice who needs to eliminate both the Byzantine Empire and Milan.
But I am not too sure about a Long campaign. I know many seasoned players can do it o harder setting, you know, the ones that complain the game is too easy. But it all depends on your experience and aggressiveness in the beginning of a game. I hope that helps :)
225 turns is more than enough to take 45 cities and hold jerusalem(for most factions, some require more).
You should have it completed within 150 turns even if you're not that good. eventually everyone will declare war on you and the only way to stop it is to kill them and so slowly you will reach the mark. --- "Brilliance! Absolute Brilliance. Someone Else in this forum with the ability to read between the lines."-King_Aragorn49
I would say 225 turns is more than enough time to finish the long campaign even if you wanted to play diplomatically or keep high favor with the pope. The only people that should worry about the turn limit is those going for total domination. I've got 55 provinces and I'm starting to feel the squeeze with 150 turns left.