The time commitment involved in an MP game really depends on the kind of player you are IMO.
The actual playing of your turn (giving orders, scripting etc...) usually doesn't take more than 30 minutes per turn (so, usually, per day) even in the late game. However, talking to your fellow players, thinking your moves 1, 5, 10 turns from now etc.. can take a big chunk of your day. As an obsessive person myself, I find that a single game can eat up my whole evening, just because I'm examining my options, testing my battle strategies in Debug games, diplomacing and so forth. Also, paranoia attacks in the middle of the night because I suddenly realize something I hadn't even seen coming and need to re-do my turn from scratch ASAP at 2 in the morning
