At the moment, just three weeks. When I came off microgynon years ago it took 3 years before I got any sign of a period. Of course being young and stupid I wasn't bothered but now it's a big deal!
Didn't speak to a doctor about lack of AF as I was in a stressful situation, 3 bereavements (1 of which was my partner) as well as my final year at uni. When AF returned, all they were regular-ish cycles (only had 3 before depo!). When I've mentioned it to doctors since, they've never said anything about it. Didn't consult doctor about coming off cerazette though, just stopped taking it.
Sinn x