I would bet nationally it is around 60-70%.
For whatever reason, at least one recruit isn't signing tomorrow "because he isn't enrolling in January," so I would venture to guess that there are probably more that feel that way.
I didn't even know what I wanted to do or where I wanted to go to college until right before I graduated high school. Even then, I still didn't know what I wanted to do. I don't blame a kid for not knowing right away what he wants to do and what college can offer him that (outside of football). These are life decisions that should take a lot of time to make and those types of decisions shouldn't be rushed.
The two signing periods can cause a lot of issues. These might not happen for hardly any recruits, but I could see them happening nonetheless.
Scenario #1:
Alabama offers you a scholarship in March. You accept and commit in August. Then don't sign tomorrow. January 25th comes along and Alabama flips a higher-rated Miami commit that plays your same position.
Now Alabama tells you that they do not have room for you and tells you to go find another school. All Bama can do now is offer you a gray-shirt or walk-on spot. You had your chance to sign and missed it. Sorry!
Scenario #2:
Your best scholarship offer is from SMU. The only Power 5 school to offer you is Kansas. You have been hearing from Iowa State, Baylor, and Texas Tech, but they have not offered a scholarship. You had a great senior year, yet no big offers are coming your way. You commit to SMU in early December, then sign on Dec. 20th.
January 25th comes around and your high school football coach tells you that he was talking to a Texas Tech coach about one of your teammates and that the Texas Tech coach told him that they decided that they had room now and wanted to offer you a scholarship, but you had already signed with SMU. You couldn't wait, so now you missed your opportunity to go to a bigger name school in the Power 5.
Scenario #3:
Your dream school (obviously Nebraska) offers you a scholarship, but says it is not a committable offer (these exist, right?) at the moment. It will depend on how their class fills up going into NSD in February on whether or not they will allow you to commit.
In the mean time, you are holding an offer from Colorado State and you really like the idea of going there, but you really want to hold out for Nebraska. You decide to commit to Colorado State in September, but you are truly waiting to find out about the Nebraska offer. Because of this, you tell the CSU coaches in mid December that you will not sign during the early signing period like they would like you to do.
Colorado State calls up another recruit and gets him to commit and sign during the early period. They call you up after the kid signs and let you know your offer no longer stands and tell you, "Good luck with that Nebraska offer!"
January 25th comes around and Nebraska calls and tells you that they have filled the position and will not allow you to commit to their offer. Sorry, guess you should have signed with CSU!