A cat eye Omni 3 claims 100 hours in steady ...
The B&M secular permanent has a 25 hour run time...
The former is enough to keep the light on even in day light during a 1200. The later will give you 3 8 hour nights worth.
Maybe if you're using tiddly usb lights that are designed for commuting, then you might have to worry about battery life. But for not that much money (esp given all the other costs of an Audax) (cat eye omni 3 is €14), you can have a light that gives you a run time well in excess of your ride length. And if you're that worried about battery life, dynamos...
In terms of rides, my record for a 200 without seeing any other rider is about 12 hours... Of which 5 were spent in the dark.
If people are worried about visibility, I would offer that pedal reflectors offer bigger bang for your buck than a flashing rear light. Ditto wheel reflectors. A nice feature of the B&M mentioned above is the integral reflector. cat eye also make one with a built in reflector called the reflex auto (TL-LD570-R), which also complies with the British standard for bike reflectors. Which is not a common feature.
Personally my rear lighting setup consists of a b&M secular dynamo on the left seat stay, a secular permanent on the right seat stay, a reflex auto on the seat post, and a pair of Omni 3's on the back of my tailfin. Depending on conditions, I'll have anything from 1 to 5 of them on. Usually only the dynamo. In fact the only time I've put all 5 on was when I accidentally ended up on an 80kph main road in Germany on the evening of Christmas day. Which was bloody stupid place to ride.
Flashing lights on group rides is just antisocial. We shouldn't need rules about, just general politeness and thinking of your fellow riders should be enough.
J