Yep, done several times as each race. It got laggy in Diplomacy so I stopped. Am hoping Rebellion brings me back.

the 8est games are when in a 3v3 or higher, the teams are so perfectly balanced that the game goes into the late stages, and the really fun stuff comes out.  2+ hour multiplayer games are the best.

I don't really play too many multi-star games, but I like long games nonetheless.

My typical game that does not include 5+ stars but takes a long time anyway is on a massive map I created with the designer available in game. I make a map with ~150 planets around a single star, tweaking planet groups and their phase line settings so that the map is not too boring or too chaotic, having some choke points but not being too linear in general. 
I've also once played on a map I designed as a 3v3v3, with 3 systems of ~60 planets and 3 players each.
I suppose that can be called a marathon game. 

I am a HUGE fan of large epic games. Something that can help (outside of picking up mods that lengthen the research tree etc like Distant Stars) is to set certain settings on slow. It seems the AI gets a touch smarter sometimes but largely it lengthens the gameplay so you do not reach the end of the research tree so quickly.

They are a lot of fun when they work out.