how long would a 1.2v 5ah battery run with a power draw of 100w?

And how would you set up the equation to find the answer?

Thanks in advance!

http://www.rapidtables.com/convert/electric/100-watt-to-amp.htm

Well P = IV, current x voltage.
5A x 1.2V = 6W.
So 100W is not achievable for any length of time.

CrossRoads:
Well P = IV, current x voltage.
5A x 1.2V = 6W.

Or in this case, 6 Watt-hours. 6/100 x 60 = 3.6 minutes if the battery could sustain this discharge, however it would have to to deliver 80 amps to do this which is particularly unlikely, even for a nickel-cadmium.

Depending on the chemistry, there may be other consequences. If for example this was a lithium-ion, it may be difficult to find where the battery was beyond the initial two minutes - after you are discharged from hospital, that is.

On the other hand, for a 12V battery, this question becomes more meaningful (about 15 to 20 minutes).