Depending on the other charges pending, it might be a while. The best approach is to be served the warrant so that any time he spending counts towards his case. However, there are some speedy trial issues. The delay before he can get to the court to fight the charges should not affect his case. If they do, that's what his attorney should be fighting against. So, depending on the nature of the charges, witnesses, and other evidence, he should have an attorney to fight for his speedy trial right, not to mention the charges themselves.
This is general informational response is based only on the information given. It should not be relied upon without consulting a lawyer and getting a full consultation. This response to the question does not create an attorney-client relationship.
Assuming your son is still in jail in another county when he was indicted for a felony in another county, you need to hire a competent criminal defense attorney ASAP.
That attorney will review the case/judgment/sentence where he is serving time and the out of county indictment. If your son is serving a sentence in the non-indicted county, they may be waiting for him to finish that sentence before transporting him to answer the indicted charge. This needs to be examined by counsel. There may be an unserved Grand Jury arrest warrant that has been lodged with the county jail where he is in custody. I really need more specifics to answer your precise question. He can file a written demand to be returned to the indicted county to answer the charges there, copy to the judge, prosecutor and clerk of court (retaining a copy for himself). This will set up his motion to dismiss when he is finally returned to the indicted county. Check with local counsel there.
I am trying to give you a general answer to your question. We do not have an attorney-client relationship by this response on the avvo website. I have not been retained to represent you. I am licensed to practice law in Kentucky and in federal court in this state and the Southern District of Indiana. You need to seek legal advice from an attorney licensed to practice in your area..