As far as the life span of the powder goes,kept cool and dry,it'll last a long time.

I've got some powder my father in law gave me thats 40 years old and is still producing great groups with normal velocity.

The powder should be dark gray/black and have a solvent like odor. IMO it smells like ether (starting fluid).

As far as using it goes,I'd likely dump all of it. Most releoaders would NEVER store powder in something other than the original can.

Being that your neighbor gave you some powder that wasn't stored that way,who knows what he did with the stuff in the cans.

The only exception would be if he gave you cans that were factory sealed.