Last fall, I resurrected an old lightset given to me by a fellow rider. Details on the set are in the post below.
After my first rides, I was a little worried that I'd under-specc'ed the battery for the setup. When I would get to the end of a ride, the handlebar light (10w, 3000mah) would go out before we hit the parking lot..... The helmet light never wavered.
Good news however is that on the most recent rides, I finished with pretty good numbers going back into the batteries, and both lights still bright at the end. The helmet light only burnt 64% of its capacity, and the handlebar light 86%. That is pretty usable capacity for the rides we normally do (around 2 hours). Unfortunately, that setup would not get me through a 12 hour race, but then again, that's not what I was expecting or planning for.
I'm glad I built the lights, as they've been really fun and definitely expanded my riding opportunities during the cold winter months with less sunlight!