First things first, try not to handle him for about 48 hours after feeding. Now back to the poop! There is really no telling how long after they eat they will poop. Some things that I've noticed about the frequency of poop are:
I've noticed that higher humidity increases frequency, also if you've noticed he hadn't pooed in like a month, bump up humidity and in a few days, POO
My snakes ALWAYS poo after a shed, and if they haven't pooed for a while, it usually means there shedding soon
Again. These are just things I've noted about my snakes. It may not be the same for all. But I would say every 2-3 weeks is average. Problem closer to 2 weeks tho.