As you're only using AA batteries, a fuse is not quite as important as it would be if you were running 48V LED lights directly off your main 48V battery.
If your AA battery pack was shorted out, it would probably just flatten the batteries without causing any other problems, but shorting out the lighting wiring on a 48V 10Ah battery pack would be a different story.
I personally wouldn't bother using a fuse with AA cells, but if you are going to fit a fuse it is always best to put it as close to the battery as possible, to protect against a short circuit in the wiring, and at the switch itself.
50mA
(0.05 Amp) sounds about right for a low power LED light, A 100mA fuse will be large enough to protect the wiring.
If the light is actually drawing 500 mA
(0.5 Amp) you would need a 1 Amp fuse instead, and depending on which AA batteries you are using it still might not even blow with a direct short, unless you're using lithium or NiMH cells.
If you don't ride the bike without your main battery pack it would save you some money in batteries if you fitted a
12V output DC-DC converter to power your LED light from your main battery pack.
Alan
