I have a brand new medela free style pump. My son is going 6 hours between feedings at night and only nurses on one side when he wakes up so I pump the other and save for when I return to work. I noticed that I have to rest for a minute then start pumping again in order to get the milk out, and even then I only get about 4 ounces, sometimes pumping that one side for 30 minutes (with compressions and massage at the same time).
I want to make sure I don't hurt my supply when I go back. I'll be working from 7-3, with a half hour drive to and from work. I could probably pump for two 15 minute sessions as well as two 30 minute sessions each day (pumping while driving won't work, because I really need to massage and compress to get the milk out.) I don't know if it would be necessary to pump this often or not. I'm sure I'll have to try it out and see, but I'd love to hear from other moms!