How long does paint last in the can?

In the can, correctly sealed it is good indefinitely. It will require a very good shaking to re mix the components but you can use oil paint that is years old.

On interior walls, oil paint lasts years. It holds up well to fingerprints and cleaning and generally only needs to be replaced when yo are tired of the colour.

On exterior walls, or furniture you should get several years. Weather conditions, wear and tear, and sun exposure will effect the durability.