My furnace 12VDC blower draws around 6-8 AMPS from my battery bank. Thats much like leaving your truck headlights ON all night. One battery will not last long doing this haha...
The other problem is if you run your deep cycle battery down below 12.0VDC and not recharge right away you will most likely do damage to your battery and have to replace it.
I would guess 3-4 hours...
I was able to run four 85AH Interstate batteries across the tongue of my POPUP trailer between the propane tanks and the front of my trailer deck. I lost one battery right away before I changed to a better smart mode charging and have run the three shown in this photo for the past five years.

Roy Ken