I'd put the chemicals in bottles when they're not being used - I noticed recently that bottled water comes in nice dark green screwtop glass bottles. They look perfect for storing chemicals in - but MARK THEM CLEARLY!!! It only takes a few seconds to pour chemicals away, and wash everything out. It's definaly good practice.

Dev keeps surprisingly well - I didn't expect it to but have had no problems keeping it from session to session. I'd always use fresh for film, and probably for doing fine prints, but for work prints part used dev can keep for weeks or even month an air tight bottle. You'll notice when it gets used up as the dev times get longer and you have to agitate to avoid blotches on the print. It's not a problem, as you can actually see it happen so you know to mix some fresh.

Stop is easy, as most stop contains an indicator - it turns red/purple when it's used up.

Fix is a bit more tricky - the prints look fine, but turn funny colours the next day. It does change colour from clear to straw, but how long that takes, and the exact point at which to chuck it is a judgement call. It's tempting to try and keep using it a little longer, but err on the sooner rather than later, as some wasted fix is less important than trashed photo's.

I'd usually put 1 litre of Dev and Fix in 8x10 trays. It keeps well enough that there's no point in using less. I use a half litre of stop as it doesn't get used up as much, and it's the most noxious of the three chemicals. There's no point in having more of it around than you need.

One of the dangers of not clearing up after each session is that spills will be left on surfaces. The problem with this is that spills dry leaving powder. These powders are massivly concentrated versions of the original chemicals, and can get picked up and transfered by touch or even carried in the air.