Games can take anywhere from 15 minutes to an hour depending on your play style but I haven't played online yet because I'm waiting for some dudes on my BUDDY LIST to pick it up, I don't like playing gams with strangers.
I'm up to Emporer now anyway and I just did the ONE CITY achievement which was fucking hard, the main problem I found was that I just couldn't produce enough of whatever it was that I wanted to win by in one city compared to everyone else who was generating mass culture/gold/tech with multiple cities.
In the end I won with the americans. Went balanced for a while to populate the city and build MARKET/BANK, got democracy and went GOLD PROD and at the end with bonuses/special people I was producing about 1400G a turn.
(I also went communism for a bit to build FACTORY/TRADE FAIR)
I really like this game though.