I only have like 500 friends
And it says this message
Do you know this user personally? To prevent misuse of Facebook, this request can't be sent. To learn more, please visit the Help Center. If you believe you're seeing this message in error, you can report the problem.

How long is this going to last?

Until you go to the help center and determine what the problem is.

In the controls, you can actually alter the number of friends you can have. Otherwise you will have a set limit.
The message can also come up if you are trying to add too many new friends at once.