That depends on your circumstances. There may be an 18 month limit in some situations.[0]

[0]https://arstechnica.com/tech-policy/2020/02/man-who-refused-...

Right, for a witness refusing to testify in a court proceeding in which they are not one of the parties.

It's different when you're a defendant or a party to the proceeding.

That's for "refusal to testify".

If you read the first paragraph you'll see it spelled out: > a federal judge held him in contempt

And if you read a bit farther, you find the reason why it was limited to 18 months:

> After losing that appeal, Rawls raised another challenge: the federal statute that allows judges to hold witnesses in contempt for refusing to testify, passed in 1970, states that "in no event shall such confinement exceed eighteen months."

This limit does not apply if you are not a witness.

Even if it did, the way we’d find out is a defendant suing the government for their release, and for that case to appeal its way up through the circuits, possibly to the Supreme Court. That appeal process would take years and would probably lose at most of the stages. During this entire time the defendant would be imprisoned.

Yes, exactly true. You can be held in contempt of court indefinitely, if it's possible for you to comply with the court order. There is no conventional "due process" because you are free to leave once you comply.