The length of Charging a SLA battery depends on the amps of the charger and the amps of the battery.
If I remember correctly the simple eqaution goes like this:
5 Amp/Hr batt x 0.5 Amp (500 mAmp) = 10 hours charge time
Is the charger you have a Smart charger? that is does it idle back when the battery is fully charged? If not, be very carefull not to overcharge SLA batteries. If they get overcharged too much, they will basically be useless and won't hold a charge for very long. I don't think there's anyway to get them back to normal after this.
If your charger is not a smart charger, make sure you watch the time closely. A fully charged SLA (6 volt 5 Amp/Hr) should read about 6.4 volts when fully charged. Before testing voltage, let it sit (settle down)for a bit after taking it off the charger.