Local cops asked how long they can detain a witness to a crime. I don't know if they can and everything I have found so far has to do with witnesses at trial. I know back when I was a cop I was told 48 hours but never knew that we had a source for that or if someone had just seen it on T.V.

I'm not an attorney, only a TX peace officer, but here is my humble opinion:

As for criminal suspects, I don't think there is a bright-line rule in TX for the length of an investigative detention, only that the police "diligently pursue a means of investigation that was likely to confirm or dispel their suspicions quickly."

I assume you're asking about police detaining on-scene witnesses at the time of the offense. I would think that the same general rule applies, namely that the police diligently pursue a means of investigation that quickly ascertains the witnesses' identities with reasonable certainty for possible later court proceedings.

I always cringe when I read "we're holding you as a material witness" in mystery novels set in Texas.

Thanks for the feedback!