There is probably a thread on this in the Polls section. You're correct, though, that there isn't a definitive answer unless you narrow the scope of the question. For example, there are minimum lengths required for a beard in certain religious observances that require men to be bearded. I don't think that's what you're asking.

In my nearly four decades of bearded life, I still felt "bearded" even when it was as short as a quarter of an inch, and that takes me about two weeks to grow. It hasn't been that short on me for a long time.

You can search "minimum beard length contest" and get a range of lengths from no specified length in one contest to 10 cm in another set of contest rules I saw. For the non-contestant who wants to grow out his facial hair, just claim the bearded life when it's obvious that you aren't shaving and don't intend to anymore. Any ambiguity is not likely to last for more than a couple weeks.

If you want to be bearded, then beard boldly, and don't get wound up over some arbitrary number of millimeters or fractions of an inch. Just let it grow, keep it clean and healthy, and enjoy the bearded life at whatever length you like.

Triibeard's Thirty-NINE years of being bearded    (The recent journey from a short- to a long-beard)
Introducing LEGO-Sheriff Triibeard     (The origin story for our polymeric polychromatic hero)
Sheriff Triibeard and The Case of the Missing Beard Balm   (In search of that scoundrel RiffRaff)
Sheriff Triibeard and the Hermit Hoarder    (Consequences of acquisitiveness)

In a world of shorn faces, it is our privilege to offer those around us the exquisite opportunity to know a gentleman with a beard.
Last Edited By: Triibeard Sep 20 15 5:00 AM. Edited 1 times.