i'm no scientist but as a rider who does them i think, and this is keeping in mind that taking that sort of pain is very much a mind/motivation over matter skill, then
- some people say dont do "intervals" more than twice a week
- some say only do them if and when you feel motivated, which for the average guy might not correspond to more than 2 times a week
- personally i prefer to do them in blocks of 2 or even 3 days in a row, then give it a break for the rest of the week, or more. for a while i made really quick gains in the power i could use, especially on the second day.
i think that provided you do some threshold maintenance work as well, maintaining it at say X% of max HR, then your threshold power at X% of max HR is going to respond as quickly as your gains in VO2max. this i noticed, more or less.
i also noticed that if you overdo this kind of "systemic asphyixa" (for fun) ... i coined your phrase from the other thread on that one because i really like it...then you will completely lose the will to do any intervals at all, drink losts of beer instead, and your performance willl suffer!