I've never had an adult corn, but I remember reading somewhere that they generally poo once per feeding. i.e. if you feed them once a week, they'll poo a few days after.
When they get older, and eat 'proper' mice the size, colour, consistency, texture and smell will change - but IIRC it should all come out in one go.
When they are fully mature they do not have to be fed as often which should mean less poos.
If it's a problem for you, you should do what a lot of people (inc me) seem to do. On day 2 and 3 after feeding give them a bath in water (slightly cool, but not cold to touch). This will give them exercise and seems to somehow relax them and make them poo!
I put Suzy in a red plastic storage box, he swims, climbs out ... then either poos over the edge into the water, or climbs back in, relaxes and poos. After he's done this (and the bulge near his vent has gone) he's OK to handle until the next feed.
This way means no poos on my lap - excellent