She started DC part time at 8 weeks, but that "part time" varied - some days I was working a full 8 hours (plus commute) so I always sent enough to do the whole day.
I would say from 2 months to about 4 months, she would go through at least 4 3oz bottles. Some days she would need the fifth, some days she would only do 3. I felt better sending more. And keep in mind if you keep it cold after you pump and in the fridge at all other times, you have many days to use it.
I should also add, that I started keeping track (on the weekends) of how often I BF her. I was feeding on demand, so I never really cared how often she was feeding. But when I looked at her normal frequency during the day, she was nursing every 2 hours when awake. So I felt good leaving 4 bottles, one every 2 hours, if I was gone 8 hours.