I find that a 7-12 day rental is substantially cheaper than a 4-day rental. I commute to the DC area each week - what I normally do is take the car upon arrival, and drop it on the second Friday... saves one weekend out of every two. I've seen no substantial price difference for a 3 or 4 week rental vs a 12 day rental on the USAA rate, except for the fact that I'm saving the cost of a weekend every other week.

I have found that HLE rentals are typically more expensive for weekly/longer rates, but they tend to be cheaper after all airport taxes for shorter-term (1-4 day) rentals. There is an HLE at Pentagon City, just a short walk from Metro, and 2 stops from DCA... it's often been cheaper than DCA for daily rentals, especially after airport taxes.

So, for me, I've been 2 rentals a month. Also note that I've found Avis to be somewhat cheaper at IAD, but Hertz to have better pricing at DCA and BWI.