At the end of January, I was demoted from Silver to Blue, missed requalification by 3000 odd miles. On 27 January I took the outbound of a F Saver trip CPT/LGW so on 28 Jan I got the email to tell me I was now Silver. On 18 Feb, I flew the return, again in F and three hours after landing in CPT there was an email congratulating me on achieving Gold.
So that was 28 days as Blue, followed by 21 days as Silver. Meteoric rise? Who has been on a tier for less than 21 days?

(now I need to fly 50000 miles before 28/2/2019 to maintain Gold, which is only going to happen if a few F savers come my way. nest trip is on J savers, that's only 15000.... )