I hown a 12000btu propane generator, the generator can run about 1h 45 non-stop per pound of gaz
(but the generator never run steady, he stop when reach 1500ppm and restart at 1400ppm, it's run 5 min. and stop for 10 to 15 min. an restart again, so the generator run for around 2h per days in my room in 12/12) So a tank of 20 pound can run for about 30h non-stop (so about 15 days for me. But it's a small room 8X6, if the room is bigger the generator will run for more than 5 min or if you used a bigger generator it's will burn more gaz per hours.
So depending of the size of the room it's could run for 1 weeks and more on a 20 pound tank, i will link you to a web-site with very good calculators, one of them: PROPANE CALCULATOR let your write the size of your tank in pound and the power of your generator in BTU, the answer will give you the maximum time your generator wil work non-stop, after you can figure better how much time it's will run for you.
The only thing it's a french site, but numbers have no language:
http://www.fernandpigeon.com