I hope this answers your questions. "Major" releases are highlighted in yellow and averaged in the far right column. The length of time between all releases is calculated in the the left Months column. I did not go back to Split Enz because, at most, those were Neil Finn EPs, not full fledged Neil Finn albums.
One could endlessly debate the decisions made here, but I'm comfortable with them. So, yeah, 35.1 months between each major release on average and 20.4 months on average between all releases.