Assuming you have a 20-pound propane tank and a generator that uses propane:
A full 20-pound tank will run most generators for about 10 hours. But keep in mind that if your generator is under 50% load, it will actually use less propane and run even longer.
So, how long your propane tank will last depends on how much power your generator is using. If you're running at full power, you can expect around 10 hours of runtime. But if you're only using half the power, your 20-pound tank could last closer to 20 hours.