Depends ......
Propane has t-stat for temp control as does electric element.
When temp is reached t-stat shuts down heating. When temp cools down t-stats closes and starts heating cycle again.
You can leave electric on....or turn on 30 minutes prior
You can leave propane on....or turn on 20 minutes prior
You can leave them BOTH on----or turn on 10-15 minutes prior
As for how much propane.
Depends on which water heater brand and size you have
Atwood uses a 8,000 btu and a 10,000 btu rated burner
Suburban uses a 12,000 btu rated burner
1 gallon of propane-----91,000 btu