Hi, I also find it very hard when my son is crying but I do try and leave him so long as the cry is not hard and basically screamy! If his cry is like a cough or winey I can leave him. I do not leave him for no more than 15 minutes though except if he quietens down and then starts up again. If he does this this usually goes on for 3 times and then he is asleep. This is all for night time sleeping though, for naps I do tend to go to him because I presume he has had enough kip and is ready to play or feed etc. If he then gets tired a short time later I will put him back down for a nap. I think it is hard for any Mum to let their baby cry though whereas the hubby can leave them for a lot longer!
Hope this has helped a bit!