Whether you timed 28 seconds or 32 seconds, in real life it does not matter. In the sim even the most anal checker won't pick the difference, and even if he did what will he do - fail you?
In typical simulator scenarios, after the second bottle is gone the fire will either go out if the intention is to continue flying, or you will still be burning if the sim instructor wants you back on the ground quickly.
In real life if the fire warning goes out - whether after the first bottle or second bottle - assume nothing. Do a fire warning system test. If it does not test there is no guarantee that the fire is out (personal experience, related elsewhere in these forums). If you have reason to believe that fire is still present, land, NOW...if that means there is only time for speedbrake armed, gear down, flaps 15, add 15 knots, cabin alerted, so be it.