How can I calculate the glide time for a B737 with engine failure?
For reference consider TACA Air Flight 110, a 737-300 that lost both engines at FL165, on approach to New Orleans in a very heavy hail/thunderstorm. 

No exact listing of how long 110 stayed in the air with no power, but the Mayday/Air Disasters show on this incident hints that it was in the vicinity of 13-15 minutes, which indicates a drop rate of around 1100-1300fpm.

A heavier example was BA flight 9 a 747 that lost all four engines due to flying into a cloud of volcanic dust at night at FL370. This led to the classic announcement:

Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.

Fourteen minutes later at FL135, they got a restart. At that descent rate, they would have had maybe another nine minutes before hitting the ocean.