My wife's estimated time kept moving as well (was decreasing over time down to 2 months, then jumped to 4, then back down to 2, so we eventually learned to ignore it), but the moment the N-400 case hit the processing time limit, and the inquiry form was available, we sent one. Interview notice came 2-3 weeks later, not sure if by coincidence or due to the inquiry.
I-751 case processing time was always way beyond that of the N400, despite an earlier submittal, so forget the I-751. it's the N400 you need to watch for and immediately send the inquiry once you hit the processing time limit.