I've never completed a full length legacy. Every time something happens and I'm unable to continue on. The one that I had that lasted the longest ended at gen 14 out of 10, but it doesn't really count because I had a catastrophic game/save crash and had to start over mid-way with copy sims from the gallery.
Its always something to do with save crashes or accidental (by a family member) deletions. I have high hopes for my current one (It'll be an Alphabet Legacy - 26 generations - with two back up saves at all times for the event that crashes/deletions happen.