Easy way to check the LB is to wire up a headlight bulb and time how long it stays on. If the bulb is say 50 watts it draws just over 4 amps (50/12) so a good 50 amp/hr LB should run the bulb for at least 80% of the fully charged capacity so say 40/4 = 10 hours. In practice the bulb will start to dim after about 70% of the capacity is exhausted. If your LB runs the bulb for 50% capacity it is OK to keep for a while - anything less than this and it needs replacing.
And Ian is right - LBs don't mind high charging currents (like 10 amps) for a while but when they are full they can be damaged by overcharging, so not a good idea to use 'normal' chargers. I use chargers specifically designed for LBs to keep them topped up over winter.
