Generally from what I've read, HIIT training seems to be the trend at increasing your VO2 and FTP.

Does anyone have any input as far as how long the intervals should be?

What about percentage of FTP - One article I read said to do 2/3rds of a minute at 110% and 1/3 of a minute at 80-90%. Didn't mind it, but you're constantly shifting.
Another instructed 1-2 minutes at 150% with equal rest.
Are there a good set of rules to follow? Basically when the word HIIT comes to mind, which is more beneficial, 110% or 150%.

Are you specifically trying to improve vo2 or FTP?

If the former, 25-30 mins of total work time divided up into sets of 4-6 mins with ~equal rest or slighly less at 115-118% of ftp are very effective.

If the latter, 30-60 mins of total work divided up into 15-30 min chunks at 88-105% ftp are wonderful (there's a big range of time and percentage you can use). 60-120 minutes of tempo ~80-85% ftp is also nice.

I think VO2 is more important, especially since I used to be a chronic smoker. But mostly because endurance will be limited to the amount of O2 you can utilize.

Thanks, I'll give that a shot, either tomorrow or the day after.

You may want to check that assumption. It's true ultimately, VO2max will put an upper bound on aerobic performance, but your sustained aerobic power will be the product of VO2max and what fraction of that you can sustain for a long period. Depending on your current fitness and level of training, it's possible, if not likely, that training the latter will produce greater benefits.

The classic idea is to periodize training: start with base, move into tempo work, then into LT work, then into VO2max work. It's classic because it works.

CTS has pushed the intervals-all-the-time idea because you do get results quicker, but as asgelle says, the results are not as good.

A third way has become popular: polarized training which I've been doing the past couple years. In polarized, one does either z2 work or z5 work, nothing in between, the z5 work being 10%-20% of total time. I periodize that too, by starting with only z2 work for a few weeks before I start adding in high end work. I do more like 10% high end, while elites do 20%. OTOH my focus is on longer rides in the 3-10 hour category rather than crits which are more the focus of CTS time-crunched programs. Polarized z5 has been found to work best with 4 reps of 8 minutes.

See Power Training Zones for Cycling | TrainingPeaks

The ones you listed are great workouts but neither is really targeting VO2max. The first is a classic FTP "over/under"; the second is an anaerobic capacity workout.

The classic VO2max workout is 3-8 min intervals at 106% to 120% FTP. One of my favorites is "3 plus 3's." Basically 3 minutes on (at 110-120%), 3 minutes off (easy spin to recover), and repeat. Early in the season, I will shoot for 4 sets and then build from there.

No guessing needed, just ride to one of these:

The training peaks guys lean towards periodized over polarized. I also think polarized is a bit silly and quite misleading when touted as a good solution for a lot of people.

In any case, as mentioned, periodized just works. Start with tempo for 3-4 weeks and work on increasing the duration with maybe 1-2 vo2 max workouts every other week, then move to another 3-4 week block focusing on sweetspot and ftp (again, maybe a vo2 max workout every other week), then moving on 3-4 weeks of vo2 or shorter, sub 1 minute efforts where you're trying to increase the amount of reps you can do, etc.

It's nice, relatively simple, and saves the really heavy intensity for when you're ready to really start bashing the pedals the hardest.

Thanks for the advice,
Based on what you said, I'll probably make a few concessions here and there, based on my personal endurance level, and how I feel about my capabilities. Otherwise, without the input I wouldn't of been able to form some new idea's as far as workouts go. I do have to look up the major differences between periodized and polarized workouts. Although I think I get it.

I think its important to mention that there is no magic threshold or v02 max workout. If you did the exact same Z5 workout every week, your progress will stall. Its important to alter the interval time or intensity each week. I like to do a steady build up of progressively more difficult workouts for 3 weeks, then take every 4th week easy by not riding much or doing any real workouts, but still staying active. Do hiking, play some basketball, ride bikes with your wife and kids, walk the dog for an hour, etc... That way my body can soak up all the benefits and heal itself for the next 3 week block.

Seems like the 30 seconds intervals with 15 sec rest time increases VO2 max power more thatn 4-5 min intervals. 10 week sturdy on trained athletes
Joe Friel - High-Intensity Interval Duration
I like to do 5-20 minutes 90% interval one day and either 30s-3minute VO2 another day with a day off between

It's generally accepted that vo2 max isn't even reached until at least 2.5 mins into an effort, thus intervals should be a minimum of that to elicit the most amount of time possible at vo2max.

Just going 30s-3min at a time is going to leave so much down time that you're only going to be reaching vo2 max at the very end of each interval, and not maximizing the amount of time spent there.

But whatever works for you. Many ways to skin the cat. I like bang for the buck myself, though.

I don't disagree but this was a 10 week sturdy done on already fit cyclist.