How long can a person be held by US Marshals without appearing before a judge?

Answer 1

Your comment to Lone Star Patriot's answer states he is being held for a probation violation.
In that case, he can be held until the END of his probation. His appearance before a judge occurred when he was put on probation.

Answer 2

I see from a previous comment this is for a probation violation. A probation violation has a totally different set of rules because you have already been through court. He can be held for whatever time it takes to complete their investigation, and then he can continue to be held for however long it takes to schedule a revocation hearing.

Answer 3

https://www.avvo.com/legal-answers/how-long-can-one-be-held-in-custody-by-us-marshall-798845.html

Answer 4

72 hours

Answer 5

The police can hold you for up to 24 hours before they have to charge you with a crime or release you. They can apply to hold you for up to 36 or 96 hours if you're suspected of a serious crime, eg murder. You can be held without charge for up to 14 days If you're arrested under the Terrorism Act.