How long in a dirty diaper? HELP!

I usually change my baby as soon as I know he has popped. 
He has been fussy and crying for the last 4 hours. On and off the boob. Crying. Won't settle. 
Just as he finally fell asleep I think he pooped. I'd hate to leave him in it but don't want him to wake as I change him either.
What do I do?