I used to do 15-20 mile rides before work; usually around 4AM. Someone had given me one of those slap lights that I wore on my left ankle and thought it was pretty effective. It weighs next to nothing and it wasn't uncomfortable at all. Of course I also had two taillights (one on the seat post and one on the seat stay) as well as reflective gear. As far as I'm concerned, the more the better when riding in traffic in the dark.

I don't recall how long the batteries lasted but I don't recall having to change them that often.