The GSS6400 internal battery will charge at approximately the same rate regardless of whether you have connected it to mains power or are using the 12V car adapter.
The charge time is independent of whether the unit is recording signals or not. However, there is a safety mechanism built-in to the unit which will halt the charging process if the battery becomes too hot. This is to prevent the unit overheating. Therefore, for best results when an internal battery is fully discharged, recharge with the unit switched off.