Paint

Depends on how good of cap seal on the jar. I've keep mixed up thinned paint in sealed jars for about 8 months to a year max. It seems my lacquers dry out quicker.

I store my thinned paint mixes in Tamiya 10 ml bottles.  I wrap around the cap with clean kitchen plastic wrap to further seal the bottles.  I've been using this method since last September, and have never had an issue.

Depends on the type and brand of paint. I love Testors enamel, but one of the bad things is it begins to harden rapidly once opened to air. I have had paint mixed with thinner go bad in about two to three weeks.  Even the stuff in the Testors bottle, unthinned, can start to go bad in a couple of months after opening.