How often should I have to add oil?

You are in trouble (unless the light is bad). If you are using that much oil, you've either got a bad leak or you are burning a lot of oil.

Valve guide seals are bad. This is a typical problem with this V-8 twin turbo engine. It's a warranty fix, else it was about $5,500. Mostly labor. The guru at the dealership is the only person to trust with this repair.