If the furnace burns 78 percent AFUE, this means that 78 percent of the heat is being utilized in the home and 22 percent is going up the chimney or flue. If the unit is a gas fireplace insert, the AFUE measurement may equal that of a heating appliance rather than a decorative appliance such as gas logs. Gas logs give off heat, but because of the opened damper in the chimney, much of the heat goes up and out. This is not an easy question. It depends on how much you want to burn it.
Some families use their gas fireplace for weekends and others use it daily.
A good estimate is about 400 hours per season. Now reduce this number to therms or cubic feet. The cost of gas is different in each city. Be sure to check with your gas utility or propane dealer. National numbers are available but they are not the same as your locality.