Just noticed in a Fairfax article about painting that they recommend that if opened, "clean the rim and place a piece of plastic wrap over the top, then hammer on the lid." Does anyone do that? I just tap the lid back on and the paint seems fine. And does paint ever go off? The only problem I've ever had is with a really old tin that has rusted and contaminated the paint.
I have also done the gladwrap thing. Seems to just give the lid a better seal. With regard to how long it will last depends on how well the lid is sealed. Most manufacturers only recommend storing for up to 2 years. I repainted one wall at my mums place last year and then found out the paint in question was close to 10 years old. Apparently nothing wrong with it. I might add it did seem to take longer to stir the mix even again.