Gestation varies a bit by breed. I've always worked on 147 days, or 21 weeks, whereas Anke is clearly more used to 154 days / 22 weeks.
I think our Zwartbles are on your system, Anke - they've finally started today, a full week after the tup went in. My fleece girls should be about 10 days behind, but the Zwarts have been so slow to get going I was beginning to think the fleece girls might beat them to it!