If you're really going to use a regulator (see Spehro Pefhany's comment that this may not be necessary), you're going to need:
- The efficiency of the regulator when supplying 100mA, so that you can calculate the current of the whole circuit
- The discharge graph of your battery at that specific current
With the discharge graph you can see how long it will take before the battery reaches the cut-off voltage of the regulator, or the 3.3V for battery health reasons.
Just to be sure, always add some margin, the circuit may draw different currents under different environmental circumstances. (I once made a GPS logging device that should run for about six hours according to the calculations and the tests - then I put it in my pocket and went hiking. It became quite hot inside the pocket, the current increased and I could log for only two hours.)
Also note that the quality of batteries degrades over time, and that its capacity may decrease.
If you would be drawing a higher current, you should also take the possible voltage drop into account.
In any case, not much can be said about the exact calculations without part numbers and datasheets.