My boyfriend got arrested on a 12 year old warrant for FTA obstruction, how long can they hold him?

If he can post the bail amount, he can get out right away. If not, they will have to bring him in front of the judge in the court where is held now, within a couple of days. Once he is done with that case, they will either release him, unless the warrant is extraditable from county A to county B, in which case they will transport him to the next jail, where he will then be seen by a judge. You should get an attorney to work with you on this. He will almost certainly get out of jail sooner and spend less time there that way.