1)What's the expected service/shelf life for
a ballast?

2)How are ballasts & fluorescent tubes that
contain mercury supposed to be discarded?

1. Depends on useage. Some of ours in the office last 2 years. Others last 5 years.
2. We through them both in the garbage.

Service life is typically several years. Replacing the florescent tubes as soon as the start to flicker will prolong their life -- a flickering tube puts a big load on the ballast.

Shelf life is pretty irrelevant. When not in use, ballasts don't wear out at all. I've used ones that had been in storage for 5-10 years, and they worked fine.

Please DON'T just throw them in the garbage. Our groundwater is polluted enough without adding to that. If your town has recycling, put these in with the worn-out rechargeable batteries. Otherwise your local electronics or Radio Shack store should take them for recycling.