Two months of actual hard work to get my multi-engine instructor Initial, and make a folder with all knowledge areas from the PTS covered. Then 1.4 month of single-engine training to get single-engine commercial addon. I took the CFI one week after the single commercial addon. It then took me about two weeks to get CFII.
For every time mentioned, at least one week was waiting for the FSDO examiner, as it was during busy season.
I took my licenses at a "serious" (-ly" milking) school though, not a ticket factory, at least not at the CFI-level.