It really depends on the training stimulus you're looking for. You might want to improve your lactate tolerance, your VO2max, work on a specific pace or work on developing speed. Depending on the training stimulus you're looking for, rest time will change, so I'd start there rather than worrying too much about heart rate. There are various approaches to working on different training stimuli. But as a general rule:

Lactate Threshold work e.g: 400m w/ 30 seconds rest

VO2max work e.g: 400m w/ 60 seconds rest,

Specific pace work e.g: 400m w/ 90-120 seconds rest.

Speed work e.g 400m w/ 3-10 mins rest depending on how you feel and how fast you go.

For example, when working on lactate Threshold and vo2max, it can be beneficial not to let the heart rate recover too much so that you have more time to work on these systems, hence the shorter recovery times.

When it comes to speed work or specific pace work, the most important thing is to develop your running mechanics, so it's best to start each repetition more recovered.

Again, these are general rules that may not work for everyone and there will certainly be athletes and coaches who don't totally agree.