The mid-length intervals work best for me. Here in Canberra, we have a couple of road hills that are 4 min, 6 min, 8 min, and 12 minutes. Hard to find something at a consistent angle that is longer than that, without being a decent ride or drive away.
All of the sections i’ve used involve turn-arounds, and it’s hard to get back within the prescribed rest period unless the hill is more than 5%.