10 lb used to last about 9-10 weeks on a 75 gal. I use 20 lbs these days, they all run multiple tanks. In the near future Id like to go up to 50 lbs
It makes sense to always go with the biggest size you can fit/use. My place charges about $17 for a 10 lb, $21 for a 20 lb. Filling a 10lb 4x a year is $61, filling a 20 lb 2x year is $42. It saves a lot money plus the convenience of half as many trips