At Saberr our philosophy is that no two teams are the same and that prescribing a cadence would be unnatural. We also heard feedback that sometimes there is a lot to talk about after just one week, and at other times the team is in a good state of flow for three or four weeks and doesn't feel the need to have a retrospective 'just for the sake of it'.
Instead, team members can submit notes whenever they like. When there are enough notes, CoachBot suggests to the team that it's time to have another retrospective. We find this typically ends up being every 2 to 4 weeks for teams.