For those long periods, just rememeber that the nominal was 3.5A; therefore it could jump when it's compressor or heater unit kicks in and possibly draw up to 5 amps.
In addition, a 2 amp draw on a 690 cca battery can drain the battery in 6 hours. Not sure the size in the Hummer, but do not believe it is in the 500 range (anyone know...guess I could check). So, just be careful if you are out in the boonies
You also have a normal parasitic load on the battery, depending on option content (more computers, more drain).