Dbthomas,
It is hard to say exactly how much just the heaters use----it is very little. We have a 325 gallon propane tank for the workshop and we have a 500 gallon tank for the house. We filled both up last November and as soon as we got the roof on the house last December, we got the heater hooked up. The tank for the shop also handles our travel trailer that we have been living in until this last June. We kept the heater going in the house during the day when we were working even though we didn't have insulation in for quite a while. We still don't have the floor insulated yet. We kept the heat on in the shop all the time. In the house, we also have a gas hot water heater and a gas dryer. In the trailer, we had gas heat and ran the fridge on gas as well as the hot water heater there.
So, all that said, we just had both tanks topped off for winter coming. Took 100 gallons for both and that is pretty much how much we used in 10 months. The winter before, we used a lot of propane and when we had the heater replaced in the trailer in November, we discovered it had a bad leak, so I can't even give you a good idea from the winter before. Ours are the unvented, infrared model REH-300T-P. We live in Arkansas around Hot Springs. We are zone 7.
Hope this helps.