Depending upon how big your tank is and how your engine is tuned a 1/10 scale usually lasts around 8 minutes. BUt, sice you say less than 5 minutes there probably is a minor problem. How is the performance of the engine, does it bog and spew oil out the exhaust? If so you are just running your engine very rich and that can be fixed by turing the HSN (The big adjutable needle on your carurator) in small incriments clockwise.
Or, you may have a leak as stated. Check your fuel tank, fuel lines, and your carburator for any leakage and get back to us.