As others have mentioned, it really varies. Most of the Trailers I've owned would allow me to go about 45 days of actual camping during the summer on a 30lbs tank. Shoulder season where the furnace would run a lot more it would drop down to a couple weeks.
Current 5th wheel can be a real power and gas hog. 42,000btu furnace, 12 gallon water heater, 18cf Norcold fridge. At first I thought I might have a propane leak being as how I was blowing through 30lbs in a couple weeks, even quicker when it was getting down below freezing at night. Seems if it's hot the fridge guzzles propane, if it's cold the furnace takes over that duty. Even the hot water uses about double what I'm used to. All that said, DW still loves this rig so call me a happy camper.