There are alot of variables which will determine how much a smoker will use. I have a Smoke Vault 24 and a GOSM 3405GW (smallest, 14" D x 16" W x 34" H).
The efficiency of the design (including horizontal w/side fire box vs vertical), modifications, ambient temps, wind, precipitation, wet or dry water pan, amount of food on the grates, amount of smoke chamber ventilation and target chamber temperature all come into play when considering fuel consumption.
My SV24 is approx. 2/3 higher chamber/cooking grate capacity than my GOSM, has a 24" chimney and mod to close the lower intakes (to improve efficiency and less grate variations) but, it burns the same amount of fuel and in most cases less than my GOSM. I ran a
turkey fryer burner in my SNP 40" (horizontal/sfb) with a full tuning plate mod for over a year, and that rig was a total hog for fuel, using almost 1lb/hr @ 225 grat /chamber temps.
I recently had just under 60 hours run time in moderate weather with my SV24. My GOSM with only an exhaust vent mod (bent tabs on the flat vent damper/disc) would run about 40 hours on average.
Most any average size vertical propane smoker should get at least 35-40 hours per 20lb tank.
If you're looking for relatively trouble-free and easy operation with portability (no utility needs), and low cost fuel, gassers can't be beat. Oh, find a local LPG distributor to get your tanks filled. The propane tank exchange outlets are a total rip-off, charging between $20.00 and $25.00 (or more, depending on where you live) for a refilled tank, plus a deposit for the first time around. I pay around $10.00-$11.00 to fill a completely empty tank.
Eric