When I used to use propane, I could get 4 full brew sessions (5 gal) out of a single tank.
Each pound of LP will give you something like 22,000 BTU. So if you use a 100,000 BTU burner at full throttle to get things heated up, you can see how quickly it will go.
I really had no problems with using propane, except the empty tanks on brew day, the cost and all of the wasted heat blowing past the kettle an onto my balls. You need less BTU output with electric because it is actually being applied directly to the medium you are trying to heat. So, a 5500W element at 18,700 BTU output is much more effective than a 20,000 BTU propane burner would be. All of that heat that you feel as you stand next to the boil kettle, is wasted BTUs, energy, $$.
The downside to electric is that in the winter, you will still be cold, even in the garage. With propane, it will be 95F in a short time.