G.K.
I'm a peer counselor with Nursing Mothers Counsel, and generally what we recommend to working moms is to pump whenever your baby would eat during your work day. It really has more to do with when they would feed vs what time it is on the clock. 10-15 minutes is a good guideline for the pumping part of your break, but it also depends on how you respond to the pump and how long your employer will allow for pumping breaks. You're going to need to factor in time to sit down and begin pumping, as well as storing milk and cleaning your pump accessories after you pump.
To get both you and baby used to the new schedule, you're going to want to introduce your alternative feeding source (bottle, cup, medicine dropper, etc) as soon as possible. We usually recommend that you begin preparing 4-6 weeks before your return date to help build your milk in the freezer, but in your case, being able to nurse your baby in the middle of the day will help. However, you're still going to need milk for his/her other feedings while you're away. You also need to prepare for a possible day when your baby drinks more milk than have pumped. Hopefully, you won't have to face this :)
Because you're going to be pumping not nursing during your work day, you also need to prepare your body. You're going to wean your baby off the breast for the feedings that will be missed. Start with one feeding, probably the 10:30/11am feeding. Introduce the alternative feeding source, and pump while baby is feeding (if possible). After 3-4 days, drop the 4pm feeding and give the bottle/cup/etc.
To help stockpile milk in your freezer, you can definitely pump after each nursing session. This will increase your milk supply, but that isn't necessarily a bad thing, especially when you need to fill your freezer. The best times to pump are right before you go to bed for the night (or right before your longest sleep), then again right after the first feeding in the morning.
Don't be frustrated if you don't get a lot from pumping. Your baby is the best pump there is, and some women don't respond to the pump as well. Depending on supply, most women can comfortably pump 2-4 oz per pumping session (total from both breasts). This amount will be more or less depending on when you pump. If you pump right after a feeding, you may only get 1 oz, but that's fine. If you're pumping when your baby would be feeding, you'll more than likely get the 2-4 oz. A lot also depends on how much your baby drinks normally. This is a great website that I recommend to moms: http://www.kellymom.com/bf/pumping/milkcalc.html
When it comes to storage, I personally recommend the bags because they are more compact, and are more easily and quickly defrosted. For expressing and storage guidelines, you can go to our website at http://www.nursingmothers.org. If you'd like more information, you can always go to our website or send me a message. If you'd like hard copies of the guidelines, I can mail you a copy of each. Just let me know.
Finally, you don't need to follow this routine at home (ie: day off or weekends). You should be able to nurse while home, but it really depends on your baby. He/she may be more comfortable with your work day schedule.
Sorry for the length of the response!!!