As of the beginning of August (which was the last time I downloaded data) touringplans.com, the official web site of the Unofficial Guide to WDW, rated expected crowd levels as follows: (I don't have my Unofficial Guide with me right now, so I can't get into the web site to check for an update)
Sat, 09 Feb '08 6
Sun, 10 Feb '08 6
Mon, 11 Feb '08 5
Tue, 12 Feb '08 5
Wed, 13 Feb '08 7
Thu, 14 Feb '08 7
Fri, 15 Feb '08 5
Sat, 16 Feb '08 7
Sun, 17 Feb '08 8
Mon, 18 Feb '08 9
Tue, 19 Feb '08 9
Wed, 20 Feb '08 9
Thu, 21 Feb '08 8
Fri, 22 Feb '08 9
Sat, 23 Feb '08 8
5 = Peak waits of 45 - 54 minutes
6 = Peak waits of 55 - 64 minutes
7 = Peak waits of 65 - 74 minutes
8 = Peak waits of 75 - 84 minutes
9 = Peak waits of 85 - 94 minutes
Peak waits means for E-ticket rides, measured by the big three mountains at MK. It is also a good indication of crowd levels in general.
It is my personal opinion that 5 days are great (the park feels a bit empty early and late), 6 days are not so bad, still very enjoyable, 7 days are still quite tolerable, and 8 days start to get a bit uncomfortable, but hey it's still Disney. I don't think I've ever been on a 9 day, but I imagine that it's pretty crowded.
So President's Day week is one of the most crowded times of the year. It is exceeded only by July, Easter week, and the week after Christmas (in that order, getting worse left to right).
Having said that, if you have a plan, and you go to the parks early and late to do the attractions, use FP wisely, take a break in the middle of the day when it is most crowded, eat during off-hours, and stay calm (and flexible), I think that 8 and even 9 days can be very enjoyable. On 9 days especially, early means before rope drop or EMH starts.
I highly recommend touringplans.com and the Unofficial Guide to WDW. You have to own the Unofficial Guide to take full advantage of the touringplans site. In addiiton to the crowd levels, the site will also tell you what parks are the best park to visit on any given day, and which parks to avoid. I have found that it is pretty accurate (I have learned that both by following it and by ignoring it!) and wouldn't plan a WDW trip without it now.