Depends on the plan, the goals of the plan and where in the progression they are.
Chad is a big believer in short/short VO2 Intervals as being a more humane way to train an athlete.
thanks @Nate_Pearson!
donāt limit yourself by age my friend! Take the āas a 55yoā out of your vernacular. āAs a kickass experienced athlete, I do a customized Bluebellā¦ā
Just my two cents! Keep crushing it!
which do you prefer doing more? which ones are more challenging? 2-3m or 8m?
that was my thought too re: tabatas at 30/30ā¦go ALL out, not vo2max.
the shorter ones being easier seems to be a common thread; maybe not a bad thing from what some literature states for adaptation, but I am still a classic style guy for the mental grit you need to bring in order to complete them!
thanks so much for your reply!
agree 100%, thanks!
thanks! I did see this blog and appreciate you passing it along. Cheers!
Hahaha. I do a customized bluebell at 115% because I know thatās what Iām capable of not because of any age restrictions 
amen dude. And FWIW, in one of my first races in 2009 I came in 13th and couldnāt believe how many 50+ beat me. I was like WTH. I hope to still be riding and active like everyone here that is 50+. Seriously, keep crushing it dude, itās inspiring.
Brendan
IMO, those are the racers whoāve got the skills nailed down and know how to use their ābulletsā. Round this way, the Masters fields are hard and ruthless.
Assuming this is an open questionā¦:
30/30 etc are mentally easy to complete and provide a sense of satisfaction. You just keep ticking them off until the full bout is done. I think this combination of useful work, relative ease of compliance and sense of satisfaction is a big driver in training plan use. I find 30/30 and variants to be a lot of fun on the trainer.
2-3 min VO2 work, assuming something like 2 min for 10 reps, 3 min for 8 reps is significantly harder than 30/30s mentally. But the intervals arenāt so long that you really enter an extended pain state or mental stress state. These are hard but not brutal and I know I can do the workout so I get it done.
3-8 min work we enter a very different territory. Mentally these are long enough where there is plenty of time to want to quit or convince yourself you canāt do it. There is a huge difference between 3-5 min and 6-8 min. 4x4 is a classic because people can do it. 5x5 starts to be an upper limit for many riders I know closely or work with. 4 x 8 is really damn hard. When folks started trying to do 4x8 twice a week when āpolarizedā became a thing, I think we saw how few folks stuck with it (different thread - no drift intended).
I think for the longer VO2 work people tend to do these as hill repeats not because itās easier to make power on a gradient, but because mentally you can pick spots on the hill as āmarkersā and use that as strong motivation. Sure you can do that on flats too but itās not as easy as on a hill.
Looking back at my notes from 10 years of power data my comments from VO2 workouts look something like this:
30/30 workouts uniformly have comments like: felt great, good workout, legs were strong today
1-3 min workouts: Legs felt good; hard but felt strong today; making good progress on the plan
4-6 min workouts: Felt good but these were tough; Not sure I was going to finish but made it to the end; really tired today; Definitely felt that one!!; and the classic: oooofffff !!!
6-8 min (typically 4 x 8): Good hill repeats, really tough workout today; glad to complete it; Just made #2 and Failed on #3; Made it through but needed full 1:1 recovery between bouts; Are you kidding me???; How is 105% so damn hard - its only 8 min???; Maybe this bike thing is a bad ideaā¦; I really suck and should take up foosball.
-Mark
The question I really have though is surely, itās better to do the longer intervals than the shorter ones.
Iād rather be able to hold 120% for 5 minutes than even 30/30s for 10 minutes. Surely the former brings more adaptions and forces your body to learn to work with high amounts of lactate for longer.
For me personally, itās about where I am in my season and what races Iāve got on the horizon. If Iām racing a crit series, Iād focus on short duration 30/15s or 40/20s. Iād be aiming for repeatability.
I just did my first crit last weekend. My love of 30/30s didnāt save me from being dropped 
Try a progression:
30/30 @ 120%
30/30 @ 130%
30/20 @ 130%
30/15 @ 120%
30/15 @ 130%
I just did Rundle tonight. I had to really search the TR library to find a 40/20 workout. I prefer this style to 30/30s or the longer 3+ minute ones. I went a little over target and did them around 125%.
Great workout though.
Edit; I got a little excited in the first sets, and had to take it down a notch or I was gonna blow!
Scoffs in coach.
Last Tuesday was 9x2.5 @ 120%. Hard, but felt pretty good.
Last Thurs was 6x7 at 105-108%. Much harder from my perspective, even though itās not much over threshold. ~40 minutes in lower V02 territory feels like itās putting some good stress in the system, but itās a grind for sure.
Iām doing sustained power build right now and Iām not seeing any of this short duration stuff yet, but I think some shorter intervals pop up during specialty.
Will try 30/15s at 120% today as when I do 30/30, Iām quite happy to do it at 150%.