How often do class dates open for 15C?

About as long as someone would wait for 31K. Forever.

There are currently no class seats. Class seats for MOS's appear when they do. You need a flight physical on hand just to request 15C/15W. There is no predictable time frame for when an MOS will have class seats, how many will be available or how long till they're snatched up. In addition those two MOS's only accept non promotable E4, so you only have until you become promotable to get a class seat.

I advise you have some backup MOS's you're willing to accept and don't wait too long because those class seats will come and go quickly as well

If retention told you it was a wait I'm sure they are correct. I do know the class sizes have been reduced to abide by Covid 19 General orders.