At work we're using a GPS mapping unit with the following:

Battery
3.7v
3960mAh

Charger
12V
0.5A

A single battery would be charged by leaving it in the device and connecting the charger.

To charge two batteries we have an add-on for the charger with two battery slots – I take it the juice from the mains is shared between the two batteries so the total charger time for both is 2 x whatever the charge time for one is.

Can anyone help explain the maths to work out the charging time?
The equipment manual meentions nothing apart from suggest a first 6 hour charge.

Also, do li-ion chargers stop charging when the battery is full? I'm worried about overchargeing the batteries if left on charge too long.

no-one?

Tried asking here?
http://www.candlepowerforums.com/vb/forumdisplay.php?f=9

presume there's no lights on the charger then?
difficult to know what's been done by the manufacturer. higher charge rate = lower lifespan of the cells. do they care more about the lifetime of the product or the headline charge time of the battery?
if it's got lights on, time it, would be the simple answer.
with the information available it's difficult to be more specific. sorry.
doubtful you're about to burst into flames though. if you were going to, you probably would do very early in the life of the product.

There isn't enough info there to work out charge times. If it is a LIon battery then something needs to be clever, either the charger or the battery or usually both. If not then it will burst into flames. Assuming it is all designed to work together then you shouldn't be able to over-charge it.