[MENTION=78894]Carbonfiberboy[/MENTION] is correct. It's not the amount of time at 90% max heart rate, it's the amt of time over lactate threshold. The problem with training with HR (as opposed to power), is that heart rate changes slowly, there is a physiologic "damper" on HR, it doesn't change instantly even though your power output might.

Here is an example from some VO2max type intervals that I did this morning. These were 3 min intervals at 115% FTP, followed by 2 minutes at FTP, for a total of 20 min. Power is purple and HR is red. You can see that power changes instantly but HR only gradually. The entire time that the power is within target "counts" towards your desired effect.



As far as what is the "best" interval length, I hear what people are saying. But I think it also depends on your overall training schedule- what you did over the last month, last week, yesterday and what you need to do tomorrow and beyond. There might be an optimal VO2max interval but it might not be optimal for your overall training schedule. Some of that too will depend on age, cumulative training, ability to recover, etc. It's a bit of an art, putting together an optimal training plan.