Good Mornin Dave,
As Bird stated, there are a lot of variables when it comes to propane useage !
I thought I might chime in because that energy form is what I use to heat my cabin in Vt. The cabin is about 900 sq ft maybe a bit more, and I have a 330 gal storage tank. I am only there a couple weekends per month, and when Im not there I leave the thermostat at 50 F.
In a normal winter, I can get by on one full tank ! I was lucky this year, I jumped on a promotional rate, $1.55 per gal, and I filled my tank early ! I also own the tank, so I can shop for the best price rather than have price dictated to me by a sole supplier !
We have a baked enamel stove that is the sole heat source, looks like a wood stove and it easily heats the whole place even in the coldest weather, other than the bathroom and the downstairs bedroom that I use electric heaters in when it gets bitter !
Good luck !