@EagleAllan Yes, the longer VO2max intervals with short rest do seem to allow improvements, while the easy endurance days allow me to recover and do my best on the interval days. I went from 270 watts to 325 watts FTP in 2.5 years most recently, coming back from a six month layoff due to injury.
In the early base phase I do short intervals, e.g. 5x and 7x 4/2, to build up my tolerance. As I improve, I move to mid-length intervals (6x 5/2 and 5x 6/2) in the late build and early build phases, and eventually get to 4x 8/2s in the late build phase. Depending on my mood, I sometimes do 2x indoor intervals sessions per week or else one interval session mid-week and then a hard (spikey power) group ride on Saturdays, with a long easy ride on Sunday.
In the specialization phase I switch to shorter high power intervals, like Ronnestadt 30/15s, which more closely resemble a CX workload. When trying to peak for CX Nationals, I will often do block training of 5 days of interval sessions (I have tried both long intervals and short interval workouts) one week, followed by two weeks with only one interval workout and reduced volume endurance sessions.
When race season comes, I usually back off of mid-week intervals in order to recover for the following weekend’s races, especially if I did two races the prior weekend.
I built a spreadsheet to track my average heart rate during intervals. Seiler’s goal is to maximize training time between 90% and 95% of max. heart rate. For the 8/2s, I usually don’t hit 90% in the first interval and only midway through successive intervals (https://www.cyclinganalytics.com/ride/346112809694), but I don’t think I could go harder and finish the intervals at the same power as when I started, so I usually end up with 13-15 minutes in Seiler zone 3 range per workout. Seiler’s twitter posts indicate that he is getting much more time than that in each Zone 3 workout, so now I am considering going to 5x or 6x 8/2s to try and approach the workloads that I would expect to see in a 45 min CX race.
I read somewhere (not Seiler) that when you can complete two successive workouts with an average heart rate 2 bpm less than the previous workout, you can increase the workload. That means you can either increase the duration of the interval or increase the work power. Watch out for signs of overtraining, though. It is still possible with polarized training, if you increase the weekly volume too much.
And you are correct, that everything varies depending on the individual.