We started with a regular monthly cycle on Lollipop in 2015. With Android6 Fairphone still could keep up on a regular basis but not always every month. Now on Android 7 the delay is stretched as the bug fixing is taking noticeable resources. The last one was some about fall 2018.
But you may read on in this issue of security updates to find out that Fairphone is/was one of the few who really managed to provide regular updates while many other big companies very often did not or just pretended by sending a fake security confirmation to their customers.