The average idle revs is about 1000 rpm. If the cc of your engine is 1600 then for each minute the fuel used would be 1.6Kx1.0K xF where F is the amount of fuel injected during each rev.
If you divide this figure into the total capacity of your fuel tank it will give the number of minutes it will run for.
You could use a more practical solution.
First empty the fuel tank. Then put in exactly 1 litre of petrol. Run the engine until it is all used up. Take the time it takes to run to empty. From your specifications regarding fuel tank capacity, say 60 litres, multiply this figure by the time taken to use up 1 litre and you get the result you require in minutes.
nb make sure the engine is at its operating temperature.