Hi A. -
First, your body produces milk on demand. So, if your baby seems hungry, put them to the breast. This also tells your body that to make even more. If you start the trend of supplementing, you body reacts by making less. Supply and demand is all it is. This is especially important to remember during growth spurts when a BF baby can want to nurse around the clock. This usually doesn't last more than a couple of days but it can make you think your doing something wrong b/c they are constantly eating.
Second important thing to remember is that BM does not "hold" as long as formula PERIOD! Simply put - - formula has more ingredients to be broken down during digestion. Hence a formula feed baby goes 3-4 hours b/t feeding and a BF baby is typically 2 hours. Neither my mom nor my sister BF and they couldn't understand why I was feeding DD so often. Additionally, it generally takes about 3 months for the babies stomach to be able to hold enough for them to go longer in between feedings.
In terms of a pump, I used a Medela Pump in Style. Like others said it take about 20 minutes. However, 2 important points about this....first, your body must get used to pumping. In the beginning, you'll get hardly anything. Overtime, you'll be able to fill up a container - but not initially. Secondly, if you pump for a while past when you are getting any milk out, you are signaling to your body to increase production (again - the more removed, the more created).
When I first returned to work, I was at a client that did not have a nursing room. Rather than pumping in the bathroom, I used my mini-van. I got the car adapter for the pump and parked in a corner. I was able to relax much better in my car than in a bathroom. There was a fridge in the building so I would store the milk there in the medella cooler.
My last piece of advice - - get multple parts for your pump. For example, if you have 3 sets of shields and you pump 3 times a day, you can use 1 set per session and wait to clean them all when get home. Also good b/c sometimes these break. Ditto for the plastic containers to hold the milk.
I wish you all the best. I nursed for nearly 15 months, while working full time. While I hated leaving my girl, it was fantastic knowing that I was giving her such a good start and even though she was in day care, she never got more than a cold the entire first year!