After 30 minutes is weird.

Generally I'd just look at how many digits they're capable of displaying. My Garmins display 100ths for the first minute, then seconds for the first hour, then only full minutes after it turns over an hour.

It was kind of annoying yesterday when I was trying to do bridge repeats and it turned over an hour in the middle of one of them - but generally if I'm doing intervals, I program them in ahead of time. At any rate I should've hit my lap timer every one, but I didn't do that either.

If you get a watch that you can program your interval workouts - and I think you can do that with most of the inexpensive ones too - do you still need to display seconds?