Fischer Clock is how you do it.
Try this:

Start 4 Days 7 hours
Bonus 1 Day 18 Hours
Max Time 11 days 1

A shorter max will definitely force a faster game. These parameters make the players move three or more times per week. Vacations and weekends have no baring on it. And, as always, the fast players can finish the game in a couple of hours. If someone quits the site, the game will end in 11 days. You can use a longer start time to give more time during a round change or series of games for matches.