How long does exterior paint stay shaken/stirred?

[walterbyrd]

HomeDepot shook the paint for me. How long before it has to be re-shook, or stirred, or whatever?
If paint needs to be stirred, what is a good way to do it? Should I take it back to the store? Put a stir thing on a drill? Or just use a stick or something?
Also, how long paint stay good in a sealed can?