I'm not an expert on this, but in general you take the mah of the battery & charge it at 1/10 of that for about 12 hours.

For example if you have a 1000 mah battery & your charger is a 100mah charger then theoretically it would charge it in 10 hours, but there's inefficiencies so you tack on a few extra hours.

I just bought a 2000 mah battery and I am charging it initially at 100mah for about 24 hours to give it a slow charge. After I cycle through it a couple times I'll charge it at 200mah which should take about 10-12 hours.

I read that you can safely leave a 25mah charger on for long periods of time, but I think a 100mah would damage the battery after an extended period of time. I have always charged mine for 12 hours prior to flight & never left them plugged in for a long time. I'm sure the more experienced on this board can give you better advice.