Here's some friendlier code that prints in the message window:
Code:
if(is(EXITRUN)) printf("\nHistory ratio: %d",(NumBars-LookBack)/BarPeriod);


I ran some quick tests on a strategy with LookBack 500 from 2003-2012 (a full 10 year period on m1 data):
BarPeriod 15 --> 16091
BarPeriod 60 --> 1000
BarPeriod 240 --> 60

I am working now on defining what I personally think is reasonable. If I said that 10 years was a reasonable period to test a 240 strategy, then I would simply use that figure of 60 to align other strategies to that figure as well, for comparison purposes:
if # is too small, add more history / if too big, reduce history