Poor battery...
But don't dismiss it as dead, see how this fits: (25 watt headlamp, 6 watt tail, 6 watt pilot, 6V system. For a 12V system, the times may almost double for a 5 ampere hour battery).
If the lights work OK, and can do so for some time with the engine stopped, (sidelights only for over two hours, headlamp/tail light for nearly an hour, assuming a 5 ampere hour battery) this tells you two things:
1) The battery was fully charged. (It won't be now!)
2) It is in good condition.
If you get a charge showing with the engine running this is quite informative too!
1) The charge system is doing something, but MAY be out of control, see 3) below.
2) if a moderate charge that slowly reduces over time (no lights, long run), the battery is most likely OK, needed a charge, and has now got most of what it needs.
3) Higher charge that does not reduce, then providing the charge system is working normally, and bulbs don't keep blowing, the battery most likely has one bad cell. If you get a high charge rate all the time with no lights, check the voltage, as this COULD be a failed regulator. If you are out somewhere and you notice this, disconnect the "F" and "D" wires from the regulator and ride home... Disconnecting will give the battery, dynamo and lights a chance!
Of course, turning the lights on, (engine off), and "rattling" the battery will possibly show any internal breaks. And if it does, then with my blessing, find something hard to throw it hard at!
Cheers,
Stan.