"Justice delayed is justice denied." It's unlikely that it could exceed 24 hours and if it exceeded a couple of hours you could get a preliminary injunction (unfortunately of course you'd need counsel to know about the detention in order to act on your behalf).

As a practical matter, more senior officials at CBP should hopefully know about their limitations and if you request counsel and stay silent then they'd likely release you.

I would like to know too. I thought Obama's NDAA allowed indefinite detentions?

I can’t find a definitive answer (and one may not exist until an appropriate court case happens) but from what I do find, it’s some hours.

At least 15 years[0]

[0]: https://en.wikipedia.org/wiki/Saifullah_Paracha