lots of variables with this question, but I get approximately .80 gph burn on my 9.9... Thats at about 2/3 throttle with a 5" pitch prop (not the standard power thrust prop). This gives a 5 knot cruising speed on my 27' - which i'm sure displaces a lot more than the 240. If I had to guess you probably would get 1/2 gph on your 240 at 1/2 throttle with the factory prop.
M