yes remember thats if the battery is fully discharged. You can get a digital charger which tells you exactly how much Mah has gone into the battery, and will stop when after automatically trickle charging when fully charged. But with some 'normal chargers' they stop charging when you decide to unplug them, if its almost dead/hardly powers your pump/s then i would charge for 19-20 hours, or there is some charge left in it and it still powers the pumps fine, but has been in use for a while, then about 15-17 hours. It depends- digital chargers work much better, and can discharge the battery before charging.
your charger is probably an on off one which charges, but you have to turn it off? or might have some leds to show something. but if you were going to use that i would say after 8 hour working day it probably gets about 5-6 hours use, so would charge for about 4-6 hours before using it again
i would suggest one of these quite cheap but automatically charges so you dont have to worry about turning it off.:
http://cgi.ebay.co.uk/Sealey-Autocharge12-Electronic-Battery-Charger-6V-12V_W0QQitemZ130168043342QQihZ003QQcategoryZ30928QQssPageNameZWDVWQQrdZ1QQcmdZViewItem however i use a Pro peak prodigy 2 charger connected to a seperate 12v power supply and this tells me exactly how much its been charged, and does any kind of battery, and charges upto 5ah, and has a digital display, and turns charging off when its finished. i also have on of these in my car, connected to the cig socket, and i can easily turn it on and off and simply plug it into my trolley, charges automatically, each time i go to different customers. About £40.