It may be difficult to prove that overblots have canonically been happening at a pace of exactly once a month, but in Book 6 we learn from Idia that there have been five overblots “in less than six months,” which he says on the same day as he overblots himself.
The Main Story will occasionally drop hints about the timeline: we know that the start of the school year is in September, and Cater mentions mentoring new students in Book 1 (in reference to the prefect, Grim, Ace and Deuce), so it does seem like Book 1 takes place in September, confirmed in a series of tweets by Yana when she also confirmed that the characters are aging in accordance with the main-story timeline.
We also know that the Interdorm Spelldrive Tournament of Book 2 is in October and Book 3 begins with the final exams of the year.
Book 4 begins on the last day of the fall semester and ends with Ace and Deuce wishing the prefect Happy New year, as they are reuniting for the first time post-winter vacation.
The VDC is held in mid-February, and the students are kidnapped by Charon “a few days later, while Malleus’ overblot happens in the spring, 6 months following the start of school.
An exact, official timeline may not have yet been released anywhere, but based on these clues we might be able to guess the following:
September: Riddle Overblot
October: Leona Overblot
November: Azul Overblot
December: Jamil Overblot
February: Vil and Idia Overblot
March: Malleus Overblot
But this is all conjecture and entirely unofficial.
Twisted Wonderland Reference Note 