To be honest, it does not really matter how long the engine can/will run. Most tanks will give you about twice the time you actually use for a flight, and even if you annoy everyone else by flying around for ever until the motor eventually cuts, the model will still glide to a landing. Setting timers is both unnecessary, and sometimes can lead to 'landing anxiety' where you just have to land, cause the timers beeped, and it might not be a opportune time to do so. Forget the thing, enjoy the flight, and if the engine does stop, (and they all do) just treat a deadstick as another thing to learn to deal with. It will make you a much more courteous and relaxed flyer in the end.
Evan, WB #12.