I've had a look at my schedules and reckon I average about 8 nights per month in a NRT bed. This obviously includes standby when I'm not called, required days off between duties (1 in 7) and simply a night between consecutive trips.
If you're weighing up the pros and cons of hotel versus apartment then, as far as money is concerned, you will end up dipping into your own pocket with an apartment (assuming you're not sharing other pilots) whereas in the hotel you would probably come out very slightly ahead of the allowance (taken over a year). However, this comment only addresses the financial side of things and does not compare the other arguments for or against an apartment versus the Excel Hotel.