[mark sr]

A lot depends on the brand/color of paint. I've had numerous buckets of paint stay well mixed for weeks but I've had a few that would begin to separate within a day or two. I always manually shake any can that has been sitting awhile. Once you pull the lid off it's pretty easy to tell if more mixing is needed. Often I'll 'box' the paint back in forth between my work pot and the paint can.

 

[mark sr]

Most paint brands also have a line of paint that isn't worth using. Basically the top end of any brand should preform well. I don't like to use any coating that is cheaper than mid line.