You can smell it as soon as you enter the room, although I would imagine that the smell gets proportionally worse with time the food has been in there digesting. The smell when it has been partially digested over 3 days is absolutely noticable. If they regurg straight away then there has been no time for the food to decompose so it may not even smell.
I would recommend against picking up your snake on the day it eats, and maybe the day after too, unless you have a need to move him/her. Lifting the hide should be fine, but is unnecessary. Unless you have reason to be worried, i.e. your snake has been ill, or it regurged the last time, or you have never fed it this size of food before, then don't concern yourself - relax and leave the snake to relax and digest too.