Hmm, not very short but about moderate length. I would say about 25-40 hours depending on how much content you do. If you really love the alchemy then its easily 60+ hours per game. There is some replayability as most games have multiple endings to get, character events you didnt see the last time, etc.

Each trilogy is seperate from the others so you can start the series with any of them, but you do want to start with the first game of a trilogy.

Steam forums are kind of dead when it comes to Atelier games, tho. For info, questions or discussion its better to visit https://www.reddit.com/r/Atelier/

I think Dusk is the "older" trilogy (it released before the others in Steam, not the oldest ever tho). If you want start with Dusk trilogy, you should play first Ayesha, then Escha & Logy, and last Shallie. About the lenght, my sessions in Dusk are about 40 hours to complete them. (the 1st and 2nd have time limit, this I think not)

The order of series means nothing, because every one of them is played in a different world, and each of them has different synthesis mechanics.