15min lessons can work well if you're focused in what you're working on. It's quite good for a moves only, spins only or power only lesson. It's not long enough if you want to do more than one thing in the lesson. I quite often pick up extra 15min lessons when there's something specific which needs working on.
I did mange to prepare moves and two compulsory dances for test in a 15min lesson, but it involved me and the coach remaining focussed throughout. I'm still impressed we managed it (it's a few years ago). However, as there becomes more to be working on, I prefer longer. Sometimes though, it would be nice to split the 30min, so you have 15min lesson, practice then another 15min as sometimes, there's no point the coach watching you go through the learning process as it's just supervised practice at that point.