How many gallons is your spa?
I have a 300 gal tub and run my smartchlor on 2. I usually get 4-5 weeks out of a chlorine cartridge. I would expect 3 weeks set on 4. If you use the Frog test strips, my experience is the strip says to change your cartridge way before it's needed. Use a reagent test to see where you are on chlorine and PH, when it gets below .5 PPM swap out your cartridge. After about 2 weeks, monitor daily/every other day to catch it before it gets too low.