Divide the capacity of the battery by the output on the charger:
24 Ah battery,
@ 1 Amp charge = 24 hrs
@ 2 Amp charge = 12 hours
@ 4 Amp charge = 6 hours.
I personally wouldn't charge a lead acid battery at any more than 20% capacity (4.8 Amps)
Also worth draining it completely every few charges, by intermittent use of the lamp, don't just put it on and leave it set fire to your house!
Am trying very hard to source some cheap Li Ion cells to make up some packs like the new DEBEN one, no weight at all in them, easy to charge, no memory effect.