The longest period I have spent in a zentai is about 11 hours. I have thought about spending a longer period, for example 24 hours in a zentai, but that needs some planning and preparation. I also have to make up some "rules" for my 24 period, because I need to eat and have toilet breaks.
Some rules could be:
-maximum amount of time not completely zipped up: 1 hour (the less, the better, of course)
-no suit changes, layering allowed
-no other rules, do whatever you like to make the time go by :)
Let's see if I have the time next weekend... :)