I've left ORS Hair Mayo in overnight before, thatwas purely by accident. I loved the results but I wouldn't make it a habit as my scalp itched from the warmth of the plastic cap and causeing a human sauna effect up in there.
A cholesterol conditioner should be fine. I've done it many times with that
I've left conditioner in my hair (Creme of Nature and also another one that I can't think of) for a few days, and I didn't notice any bad side effects.
I didn't see any tremendous benefit either, so I don't leave conditioner in my hair now for longer then, say, 30 minutes (or however long it takes me to detangle my hair)
I sleep in conditioner some times...shoot, I've fallen asleep with conditioner on my head AND the soft bonnet hair dryer on my head and running... I didn't have any problems.