This is not a straight forward answer. There are lots of variables to consider. However, you can estimate the amount of time your heater would be able run at full tilt with a given amount of propane.
Use these estimates as approximations. Don’t let your tanks run low.
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For a small wall heater or floor model you could estimate using it at full volume for up to 18 hours with a full 30lb bottle.
For a larger, installed wall heater that is rated at 36k BTUs, you could estimate 11 hours at full volume. Expect shorter use times because the heater is putting out more heat, burning more propane.
If you have a tall, installed wall heater that puts out 70k BTUs then you could expect less time on that same 30lb bottle, with the heater turned up all the way.