We've tested a GTKPRO2 that was stationary/not-moving and found a fully charged internal battery lasted approximately 3 hours.
Tests were conducted in good mobile coverage areas in a non-moving vehicle until the battery was 100% depleted and no longer talking to the server/network.
For best results, we encourage you to keep the internal battery connected and charged, and the vehicle battery connected and fully charged.
While we are unsure how someone will cut the power cable to the tracker without triggering the motion/movement sensors, if you are really worried about this, we suggest you create a battery level Sensor trigger and be alerted to this unique situation. Unknown faults/shorts in the electrical system can and do happen, so knowing about them has to be good. This would be quite useful if you find yourself jump-starting the car more than once as there could be a problem with the charging or battery itself!
Note: Other tracking hardware will not last this long. Mobile coverage will also determine the life of the battery as the harder the transceiver has to work the more power it will consume.