So what's the Patch Schedule like?

do we just get one massive patch every 3-6 months that adds all the content and fixes things, and nothing inbetween?

or do we get smaller basic "fix" and "minor content" patches here and there, and then Major Content Updates every couple of months?

or what?

The later: Yoshi-P himself stated a while ago that he plans to do major content updates every three months or so which are meant to be kind of like mini-expansions, while bug fixes and minor patches are done semi-regularly in between as needed (usually these are called 'hotfixes' and are often preceded by emergency server maintenance, such as the imminent maintenance to fix some issues that came in with 2.1).

So yeah, your second statement is correct: major content patches every three months, and emergency patches/bug fixes in between when necessary.

we're getting 2.15 in January, no idea what it will add yet but probably some fixes, balances and maybe a few new features

They do have routine maitenace every monday. That covers routine fixes if that's what you wondering about.