This page suggests 0.1%: http://www.therecoverycenter.org/resources/weed-through-the-...
I'm not sure to what extent that's a political advocacy organization, so let's just call 0.1% a lower bound. The absolute upper bound is ~5%: https://twitter.com/JohnFPfaff/status/985996204986241026 But that assumes that everyone arrested for marijuana possession (even if it was only one of the charges) was incarcerated. But that's absolutely not the case. Rather, marijuana arrests are almost always pretextual and used to intimidate; basically, catch & release.
There sure are a ridiculous number of arrests, though, and these days having any kind of arrest on your record can be a significant handicap.