A You should put a liner on at least the base (but with holes lined up with the drainage holes). A liner on the inside may help prevent the soil drying out. You might consider spraying the inside and outside with apple cider vinegar to inhibit growth of fungus. Advice is mixed on whether you need to varnish it.

A Whether or not to line your barrel with plastic is entirely up to you. Many people feel that a liner significantly extends a barrel's life -- though how much is questionable. One way or another, if your barrel is solid oak, it should last for a good long time.

A about 10 years
I'd say that a whiskey barrel exposed to the elements lasts as a plant container for about 10 years at most. My idea was to make them pretty and decorative, and hopefully prolong the deterioration of the wood.