Try to work within your VO2 max for long intervals so don't worry about reaching 400 watts in your intervals at this time..
This gives more time at VO2 per interval and per session and is a superior way to approach VO2 max training. Remember, it takes about 1.5 minutes before your body is even at VO2 max so short VO2 max intervals are not the best way to approach this type of training.
So the 4-6 minute range is good and 5 is ideal IMHO.
20 -30(max) total minutes is enough for most and in fact the most that most men can do correctly.
If you do short VO2 intervals then shorten the recovery periods to less than the interval length so that you reach VO2 max quickly with each interval...but I think you'll see your best progress with long intervals.
For optimal progress block train the intervals for 3 days in a row....BUT YOU MUST take at least two full recovery days after each block and probably three. Total ride volume for the week MUST be cut SIGNIFICANTLY as well.
Try
Day one.....5 X 5
Day two.....4.5 X 5
Day three...4 X 5.
1 to 1 work to rest ratio....and really easy between intervals.
If you cannot maintain the power on ALL intervals then STOP the workout...you are toast and need recovery. You may see this on the last day of the block so be aware.
Heart rate will drop on each successive day due to fatigue so be sure to ONLY train by power. Do not try to push you heart rate up to day one level.
If you do not block train them then try to do them twice a week for 5 X 5, but always after a rest day or recovery day.
Never do sprints, threshold or anaerobic intervals before VO2 training. You can do them on the same day but only if you are doing twice a day workouts in which case you should do the higher intensity work first and allow for at least 5 hours between sessions. ie in the am do sprints and then in the afternoon do your VO2 work. However, when FOCUSING on VO2 for OPTIMAL results it is usually best to just do the VO2 work and then some threshold, Tempo or L2 after.
I have my clients do most of their VO2 work on a trainer for the totally controlled environment. Cadence should be around 110. Hills are also good with a cadence of about 85.
Remember, anyone can train hard and very many do, but VERY FEW allow for optimal recovery so the body can supercompensate and get stronger. So do not get volume crazy in your training week when doing a VO2 focus.
Good luck