Ticked over the 100SC mark on a Monday evening, and was upgraded to Gold by Wednesday lunch in the same week (Gold wasn't there when I checked at breakfast, but was there when I opened the app again around midday).
'Membership month' hasn't changed, so I have Gold until April 2022 (being the rest of my current membership year, plus the next year). In retrospect, I should have timed my challenge to start as late as possible and to complete it right at the beginning of the new membership year, but with travel being so unpredictable (and over 50SC of travel already planned for January, before I booked more), I just wanted to get it done and secured, knowing I'd already be half way there if I timed it for now!
Of course, there's no jump start to Platinum - app shows I need the full number of SCs (minus what I've earned this membership year) to reach it... just in case anybody was thinking this challenge would put them half way to Plat, which would be the case when earning Gold the 'normal' way.