My case:
with 302 SD movie recordings averaging 1889MB and 114 minutes not including filler for time
and 427 HD movie recordings averaging 5686MB and 119 minutes,
you should get more than 150 mixed movies per full 750GB drive.
BTW, I just added a 1000GB Cavalry drive to my 5 750GB WD drives so they are no longer all full now with one dedicated to series and PBS types.
Note a 750,000,000,000 byte drive holds about 697GB.
The largest SD contributor is 183 TCM recordings--sure wish it was in HD.
-Ken