Personally, N=1, and probably because based on my training history maturity, but break through a threshold of ~13 - 14 hours a week makes a step change in my performance. The mix being mainly Z2 and low tempo with about 20% SST and ~2-3% over threshold. NB: Not referring to Build or Peak.
If your background is 5 - 6 hrs per week you might see a step change around 8 - 10 hrs of mainly Z2 with some intensity.