I don't know for sure if it has a shelf life or not I only know what I did. I had some that had probably sat for the better part of 10-13years. Still new in the box the PO gave me with all the other bits and pieces he'd bought for the boat. I ended up using it 3 years ago and had no problems. However the guy at the paint
store was pretty shocked when he went to shake it up for me
It nearly destroyed his paint shaker
. I had suggested that he put it in the slow rolly motion one...
but hey he didn't know anything about boat paints! Any who the paint worked fine for me, no exagerated growth or problems with it adhesion ect. In fact I haven't repainted in the 3 years since.
If you don't have a paint guy you want to piss off I'd get one of the paint stirrers you chuck into a drill and have at it. However you'll want to either put it into a bigger bucket or pour half of it into another container and then mix it all together at the end.