About a day is the limit as far as I can tell.

I often work at night, go to bed and finish in the morning. I haven't had any problems with doing it that way.

I also cover my trays. Just use an empty tray, upside down.

If you're just practicing or doing routine work, I see no problem with keeping your chems overnight but, if you're trying to do your best work, just replace it.

Dektol starts to change color and get darker when it's beginning to expire. If you let it go completely bad, it will look like coffee.
Just watch the color. When it starts to get darker, that's your signal.