Not entirely sure what you mean by release schedule. I found this[1] list which lists releases and dates, but (unless I'm being thick,) it's not like they're minor-bumping every x months, it seems to be arbitrary.

And if it's arbitrary, I don't see how it matters. If I'm looking up the release date for 6.3 then I may as well look up the release date for 2.6.32...