If you can stir it into a usable consistency then you should be ok. However, make sure all the stuff that has settled in the bottom is properly mixed in – it can take a lot more stirring than you imagine. Scrape a screwdriver, or something similar, along the inside-bottom of the can, if it isn’t mixed in you’ll find thick deposits there.
How long an opened tin of paint will last depends on a lot of factors. How much paint was used, how much air is the tin and so on. Impossible to say really.