Do you have dual propane cylinders?
Valve one in service......flip lever so it points to that cylinder.
When that cylinder goes empty..regulator will swap to other cylinder/indicator flag will turn from green to red. Valve in other cylinder--flip lever to it.
Valve out empty cylinder, remove it and go get refilled.
OR valve both cylinders in..........check indicator flag routinely cause when it turns red BOTH 1st cylinder is empty/regulator has swapped to 2nd cylinder.
Flip lever to 2nd cylinder.flag goes from red to green BUT----2nd cylinder may be full/partially full/almost empty cause you won't know when it was swapped to.
After a while of using them you will get an idea how long YOU can go on one based on YOUR type of usage.
We typically go 6 weeks on one of our 30# cylinders.