This depends on both the size of your generator and how long you plan on running it for. For most home usages we recommend either a 120 gallon tank for short outages (1-2 days) or a 500 gallon tank for more extended time periods (8-10 days).
Here are more exact estimates based on the size of your generator and assuming an average 50% load on the generator during the outage.
22kw Generac: 48 hours on 120 gallon tank / 197 hours on a 500 gallon tank
20kw Generac: 50 hours on 120 gallon tank / 210 hours on a 500 gallon tank
16kw Generac: 59 hours on 120 gallon tank / 246 hours on a 500 gallon tank
7.5kw Generac: 138 hours on 120 gallon tank / 574 hours on a 500 gallon tank