We've tested a GTKPRO2 with good mobile coverage while driving on a freeway, it lasted about 44 minutes (unplugged from all wires while driving).
We anticipate inner-city driving or in areas with less dense mobile coverage the power consumption to transmit and the greater information density transmitted should consume more power and therefore decrease the life of the internal battery. For best tracking, we highly encourage you to keep the internal battery connected, and the car battery fully charged.
For other devices, the battery life will be considerably less.