I looked into it for a Honda 2000 - the 20# propane tank will supposedly last about 24 hrs.  1 gal of gas last about 10-12hrs.  Figure 2 gallons of gas will do what a gas grill tank will.
Propane stores longer, gas not so much.  We store a couple jerry cans and every quarter or so will use em in the car or boat, refill it, use stabil.  Also run the genny about every 3 or 4  months for a little bit to keep the gas fresh.
Figure you get a 100# propane tank.  That's equivalent to 2 5-gallon jugs of gas.
We did 7 days on 10 gallons of gas (on and off use) - kept fridge, chest freezer cold - had TV and fan in evening and slept with a window shaker AC in bedroom.  This was after a hurricane.  Would fill the tank up after work and run til we left in morning, say 6 pm to 6 am.
Anyway - more details are required - size of genny and desired use.