Well, it’s not like the intervals will stop working, at least when it comes to fat loss. So you don’t always have to progress in terms of variables.
If we were talking about anaerobic capacity and lactate tolerance, yeah, we would need to progress toward more intense intervals. But if your goal is fat loss you can stick to the same type of intervals.
Now, I don’t know if you meant 30:90 or if it was a typo and you meant 30:30 intervals.
30:90 is more like the “sprint 8” method where you alternate 30 seconds of “sprinting” with 90 seconds of complete rest for 8 sets.
I will assume that you really meant 30:90.
Anyway, as I was saying, if your main goal is fat loss, the intervals will not lose their effectiveness even as you get used to the duration and work:rest ratio. You will still be using energy to fuel your sprints and you will still produce lactate. Fat loss progression will be similar.
It might slow down at one point, but not because the method loses it’s effectiveness but because regardless of the method/approach, fat loss slows down as you get leaner and leaner.
That having been said, I normally use a progression with intervals. Not so much because the body adapts to the method and that it loses its effectiveness but rather because intervals are very traumatic on the body when done properly and most will need to work up to the full regimen, otherwise the stress might lead to recovery issues.
With normal intervals, I progress by gradually increasing the work: rest ratio. For example, going from 15:45 to 20:30 to 30:30 to 20:10.
If you are using the 30:90 protocol you can still play with the work: rest ratio by decreasing the duration of the rest period. You could go as low as 60 seconds of rest when you are really well-conditioned. But remember that with this method, the goal is to have the best average performance on the sprints; if you decrease rest periods so much that performance goes down, you are going against the purpose of the method.
When you look at the sprint 8 protocol, on which the 30:90 protocol is based on, they simply progress by trying to improve performance on the sprint sessions over time (they normally use a special treadmill, bike or elliptical that calculates the distance reached in the 30 seconds).