Since there's a limited number of things you can do on any given turn, and since the multiplayer can be done asynchronously, everyone can simply agree to a rule like "we play one turn each morning and one each night", and then (after a single synchronous night getting past all the ultra-short initial turns) there's no way to spend more than 30 minutes or so a day on the game. Granted, each game takes a few months...

This is the purpose of Freeciv's variant Longturn. (Freeciv being the open-source game inspired - mostly - by civilization 2.)