I just did some more research. As it turns out, ponies don't activate bitch mode every ~30 days like humans do. Instead, as long as it is season (read: NOT fucking March), they cycle about every three weeks.
That in mind, Midnight had her last period exactly 18 days ago. Consequently, she should get the next one around chapter 75. How oddly convenient for reasons that you wouldn't understand.
Then again, she doesn't seem to show any tendencies of actually doing an estrus. I wonder why that might be.
And to balance out the SRS,
Lube or lube not. There is no dry.