I'm freelancing, and my goal is usually to work 12 productive Pomodoro's every day, i.e. about six hours of productive work. That really feels like I've extracted all the productivity I have in me for a day, and still leaves plenty of time to rest and do whatever I want. (When I used to work at home I would e.g. go for a run after four or eight Pomodoro's, and continue with the rest afterwards.)