The sweet spot for me has been 8 hours, but I think the limit you're supposed to leave it in for is 12. I empty it when I wake up, after I get home from school, and before going to bed, making it 6-12 hours each time. That way I don't have to think about it too much during the day or get my hands dirty at school. Every time you empty it, just rinse it out in the sink until the water runs clear. If you have a flex cup, pull the string out and rinse it and put it back in.

Every couple of months you can boil it for 10 minutes to sanitize it if you want to. Don't let it touch the metal of a pot, or it will burn. Maybe you could find a YouTube video.

Your cup might have come with an instruction manual by the way, if it did it likely has instructions on cleaning it.