I lay awake last night fretting over not having started the next muse-letter or not even having an idea in mind for it. Before launch I described it as a semi-regular thing that would not be sent "according to any strict timetable" yet I put undue pressure on myself to meet one.
The first three letters have been sent out bi-weekly but that was never the original intention.
While it's common practice to be regular with these things I don't see semi-regular as an issue, rhere's no point mailing just for the sake of it and filling people's inboxes with rubbish.