My experience has been the big drops have come about 3 weeks to 1 week out, at least for the typical car classes. If you can book the car service and back out without penalty, go ahead.
One thing that I've done in the past is to simulate a car rental by changing dates to see how the rates are behaving. For example, my actual rental period is a Thursday in mid March at noon to about 6 PM the following Wednesday. Using just Alamo with no codes and no prepay and a compact car as an example, I start working backwards, moving the rental back a week at a time, but always keeping Thursday at noon to Wednesday at 6 PM. Here's what I'm seeing this morning:
Actual dates $507
1 month out $507
3 weeks out $507
2 weeks out $507
1 week out $221
This week $274
In this case, the best rates are about one week out, which is consistent with what I've seen before. I should point out that this is just a guideline. In reality, I would expect a higher demand in mid-March than the last week of January, so that could affect things. I should also point out that in some of those tests some of the other car classes were less expensive than Compact...I just stuck with that to keep things consistent. Bottom line...keep looking, but once you get inside that two week window, really look.