The goal of VO2 max training is to keep the heart rate above 95% for as long as possible; this is best achieved by keeping the rest interval relatively short. I find intervals of 3-4 minutes with 2 minutes rest common. Shorter intervals - like 400m or 600m - might have 60-90s of rest.
Something I routinely struggle with is the idea that you want to run the intervals as slowly
as possible while still achieving the desired intensity. For example, getting up to 100% HR Max with a 3:30 km is less desirable than achieving 97% HR Max with a 3:45 km because you can run more 3:45 km intervals. The numbers are made up, but you shouldn't run as fast as you possibly can.