How long to charge a Lead Acid Battery?

I got a s350 scooter from a friend, there was no charger, so I found one that goes to a scooter, but I think it is for a larger battery application. My question is, when the battery starts bubbling, do I turn it off then or let it boil for awhile. It seems to bubble pretty fast, within about 45 minutes, after an hour it is boiling to the point that you can hear and smell it. I keep a close eye on the water level,(have a jug of distilled water handy) but I'm not sure if putting more water in it all the time is a good thing. Can this thing really be done charging after an hour or less? Thanks in advance.