it's not about making the interval shorter, it's about making the interval a
reasonable length of time to get the adaptations your are looking for. 20-30min is a reasonable length of time for a long interval... VO2max 2-5mins, for anaerobic capacity about 1 minute makes sense, NMP 5 sec etc.. and i think you should stop thinking about it in terms of to improve my 1hr power i do a 1hr interval.. if you road for 3hrs at an endurance pace you'd also be making adaptations that would impact your 1hr power... it's the underlying adaptations you are looking..
interval training allows you to ride more total time at a higher intensity than if you did a single uninterrupted workout... full stop.
if you want to work at an intensity that on a perfect day with lots of rest you could do for 1hr... then if you did that intensity multiple times for only 20min you could do that intensity more easily at any time in your cycle... one caveat.. with threshold training you do need a min time in to effectively train ~8-10mins but beyond that your good... VO2max i think the accepted number is 2mins
if you want to do an intensity that you could do for 1 minute then likely you could do that intensity or close enough to it to any old time so, it likely makes sense to just keep do 1min... but again if you wanted to do 10 total minutes at that intensity.. it would be impossible to do it for 10mins.. so you use 10 repeated 1min intervals to get more time at that intensity..
if you're working on neuromuscular power.. 5 second.. makes no sense to shorten it to 2 seconds...
i think you may just be over thinking it... what you want to do is get more time in the intensity you are after to get the adaptation you are after...
[edit] one more thing... the thing you give up in interval training is time i.e. added is the recovery time between intervals. to minimize this, you likely want to make the intervals as long as is reasonable i.e. less time to do the total duration at the target intensity you are after and then you are making more efficient use of your time.
have a look at this chart... look at "Magnitude of adaptations of by training level" you'll see how much overlap there is in terms of adaptations when training a certain levels... except for NMP and AC the levels only more or less affect this or that adaptation.. so are more or less a better uses of your time for x or y adaptation.
http://www.freewebs.com/velodynamics2/traininglevels.pdf