I have a group of friends who come back to a saved game. Our first game took about 5-6 hours, without a turn timer. Turn timers would help, but I'd still estimate 5-8 hours for a full game on online speed.
The multiplayer scenarios are much more doable for a single sitting. They are 50 turn scenarios, we did one with 90 second turns and it finished in under an hour and a half. Doesn't quite give the full experience though.