How long paint lasts depends on how well the lid is sealed. Most manufacturers only recommend storing for up to 2 years. I repainted a wall at my Mum’s place last year and then found out the paint in question was close to 10 years old. Apparently nothing wrong with it. It did seem to take longer to stir the mix even again. - darylhewston
Oil based paints have a shelf life of anywhere from 2 to 15 years. Depending how well it was sealed and stored.
One of the best ways to tell if your stored paint has gone bad is to test it on a piece of board or timber. If lumping occurs it could be that chunky sized bits of paint have formed despite a vigorous stir. Extreme bad smell like rotten eggs might be highly concentrated.
Even then if it passes these tests the paint itself could be compromised because of mould growth or contamination from the container itself. Sometimes the integrity of the paint has been compromised and does not last as long as you would like.
I recommend getting new paint just to be safe. You may bring your current can to the shop to allow the paint specialist to colour match the paint in the can. - redracer01
I'm all for using old paint if it's possible. But recently, I was stirring up some 6-year-old acrylic paint that was full of chunks. I decided not to use it as I couldn't blend it into a smooth consistency. If it looks like a portion of your paint has solidified, the portion that has remained a liquid is compromised to some extent. Unless you were using that on something that is not important, I would encourage using new paint. - MitchellMc