My friend violated his probation for the first time. He's now in jail and has not gone before a judge yet. I want to know how long he's probably going to be in jail.

Depends on what he did. He can get as much as the full remainder of the probation behind bars. Unless there are new charges pending.

Hes just on it for un paid fines. And his probabtion was up a couple Months ago. BUT he just wasn't "Caught" yet.

Was paying the fines part of the terms of the probation? Or are they unrelated to the probation?

Really need a question and follow up with more information.

But there is no way to tell what the judge will do.

OP might want to supply the following info:
Original charges for which the person was convicted;
Sentence imposed (jail time & fine); and
Sentence actually served or paid.