For some numbers to chew on:
I used a 29 gallon tank up until a month or so ago. I use a 5 pound tank of CO2. That CO2 lasted 10 months, running about 3-4 bubbles per second measured at the bubble counter. A 90 gallon tank would use about 3X as much CO2, so it should last about 3+ months. You can't go wrong getting at least a 10 pound tank.