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%.