CH4 (or methane?), what is a typical reasonable time? I've often wondered how long they can detain someone...
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In a
Terry encounter, the answer is, long enough to confirm or dispel the suspicion of criminality. If the State has a reasonably good LE computer system, and/or adequate training, the added time in a traffic based vehicle stop should be about impossible to discern. Some states have primitive/slow/crummy systems, and some have subpar training, neither of which is excusable.
In this State, WAPA (Washington Association of Prosecuting Attorneys) does a LOT to improve the training on such issues. The change in the quality of training on this and other issues as a result of Pam's (our staff attorney and all around first rate nerd) efforts over the last decade has been truly impressive.
You can go to "http://70.89.120.146/wapa/manuals.html" and look for the 2015 (latest edition) Search and Seizure Manual - it's the fifth one down on the list. Don't print it - put on your desktop if you really want it, as it is a 435 page PDF. (I do, and teach our deputies to do so, too.) For those outside Washington, some of our search and seizure rules will be more restrictive on LE than yours because of our state constitution. That provision (Article 1, section 7) is more protective of privacy than the 4th amendment.
For a good general discussion of
Terry issues, go to your favorite search engine and search for "Routledge Doug Mitchell". I know the authors - ugly as can be, but great folks and really smart.

It is designed to be approachable but valid for a broad potential audience, from cops of all ranks, to prosecutors and LE legal advisors and private citizens who want to be more informed. The authors are also available to do day long seminars for the same audience(s).

If you are really crazy, you can spend the time and money to get a copy of LaFave's search and seizure treatise, but it's several hundred dollars and 6 volumes with the index. The portion on
Terry is well over 700 pages in a small font.