I think Sephex nailed it for me, I'm fine with any length as long as it is engaging and doesn't feel padded. One of the things I actually loved about The Last Story after coming off putting in over a hundred hours into Xenoblade Chronicles was that it was short. Like 20-30 hour range and I did most of everything I could in a first playthrough. It was odd playing a modern JRPG that was the length of the SNES era games but honestly it felt right and I never once got bored or felt the need to over-analyze everything cause I had too much downtime with prolonged cutscenes or fetch quests to give me time to ponder. It just felt right.
Time usually doesn't bother me as long as I am enjoying the experience. I whine about not having much free time but I sometimes find ways to pour 50+ hours into a game I enjoy in the span of a few days. Still, cause I am busy, I do tend to gravitate towards shorter games. I haven't touched AC4 for the sole reason that I just don't have the freaking time to play through it and get my other stuff done. Handhelds are the best for giving me some leeway.