Let’s say you want to take your TiZ (per workout) from 40 minutes to 90 minutes (as you might if you did a 30-minute interval in each hour of your long ride)
That’s 50 minutes to add, and it’s probably sensible to add (on average) about 5 minutes per workout. I make that 11 workouts.
Depending on the training density and how you recover, you may need a slower progression. At twice a week, you’d be adding 10 minutes per week to your interval length per week. At three times a week you’d be adding 15 minutes per week and that might be rough.
Since individuals can hold FTP for different durations (seems to be accepted here it can be anywhere from 40-70 min) you may want to quantify your effort differently, or at least identify how long you you can hold your FTP. Intensity of 99% FTP is essentially race pace for that length of time. In my experience one effort per week is sufficient.
I also know that many focus on time in zone as the key metric but I think recovery should not be overlooked. Granted this was a couple of decades ago, but going from college to training/being around professionals, I was surprised how short & intense recoveries were. They literally trained their bodies to recover while maintaining tempo pace.
Another thing I noticed… the only time we broke up threshold/sweet spot/tempo efforts would be super-threshold. The longer the duration, the pace would decrease (i.e. sweet spot or tempo, though we called it differently). Ultimately it comes down to fatigue management. How many race efforts can you maintain a week before breaking down?