While running a T3 with a TomTom BT unit....2 hours was max but don't forget your probably going to be in a car where you should be able to use the Cig adapter for power for the T3.
If your contemplating using a BT config system for GeoCaching/walking, go peruse the iQue 3600 forum - you'll find that most geocachers use a dedicated GPS device that's designed for such things rather than trying to merge a device that's not really designed for.
Also, if your planning on using the T3 for in car navigation - I'd really suggest saving a few bucks and just getting a serial cable connection instead of BT......at least the TomTom unit was correctly configured with Serial Antenna option and their car kit for being able to power both devices simultaneously.
If you want to review my poor review of the TomTom unit - do a search with in this forum for TomTom to see the specifics.
I still have my iQue 3600 and also use a Garmin 2620 for most of my in car navigation needs now....the screen is much better for overall day and night viewing then either the Que or the TomTom with T3.