Do you mean how long once the skin has actually started to come off?
In a habitat with proper humidity (assuming the snake is not dehydrated), the entire length of skin from head to tail should come off in about 30 minutes (starting from the moment the skin first starts coming off at the head), and it should come off in one single piece. However, if the shed is incomplete and comes off in multiple pieces, there is really no time limit. It could take a couple hours or it could take a month before every little piece falls off. You should offer food just like normal during this time and focus on fixing the humidity so that the next time he sheds, it comes off in one single piece.
IMHO, nobody should ever have to mist a ball python's habitat unless there is something inadequate about the cage design. A good cage design will maintain adequate humidity levels 24 hours a day, year-round, without regards to whether or not the snake is in shed. You mentioned that the humidity in the cage is usually 40% and you mist it to get it up to 70% but then it drops back down in an hour. I would highly recommend using the tub method instead of a glass tank if you're having that drastic of a problem. 50% should be the bare minimum acceptable humidity, 60-65% is better, and that should be maintained all month long, not just before a shed.
As the previous poster mentioned, I would immediately disregard anything that that pet store employee told you about ball python care. Under Tank Heaters should be used under the tank with a thermostat (and preferably substrate), and this will prevent burning. 24-hr belly-heat is the preferred type of heating for ball pythons.