Sony Ericsson T637

I received a Cingular-branded car charger with my SE. When you have it plugged into the car, the Cingular logo lights up orange.

I did not receive any instructions with this 'free' charger.

I cannot find anything in my phone guide either.

Can anyone tell me how long I should allow the phone to be on the car charger?

Thanks in advance. 🙂