Hi A. -
A little background about myself: I'm an OB nurse and I've been involved in La Leche League for a number of years. In addition, I breastfed my twin daughters for almost 2 years.
First of all, congratulations on your new baby & on being so dedicated to breastfeeding. I know it can be a lot of work, and I can't imagine what it's like to have to go back to work when she's so little.
When I'm counseling new moms about breastfeeding, I don't stress watching the clock during a feeding. It used to be that women were told to nurse baby for 10 minutes on one side and then switch. However, we know now that the milk a baby gets during a feeding changes from the beginning to the end. At the beginning, the milk has a higher water content which sates the baby's thirst. By the end of the feeding, breastmilk has a higher fat content. This is called "hind milk" and is where baby will get most of her calories.
The best recommendation I can give is to let your baby nurse on one side until she is satisfied and pulls away (or falls asleep). Then you can offer her the other side, and of course, start the next feeding on the second side. If she doesn't want to nurse on the other side, that's just fine. My daughters always nursed on one side only, since I nursed them at the same time, and while we had long nursing sessions (45-90 min), they gained well and never needed supplements.
As far as pumping and getting an adequate amount to feed your baby while you're at work, there are several things you can do. You might try pumping one breast while you are feeding your daughter on the other breast. Pumps are wonderful, but are not as efficient as the real thing. By pumping while you nurse, you will receive the stimulation from your daughter, which might increase your ability to produce expressed breast milk. If you don't have better luck with the breast pump, you can also try expressing your milk manually. Some women respond better to hand expression than breast pumps.
Did your breast pump come with a manual attachment? If so, you might try using that in the shower. Warm water can help with let down and increase the amount you are able to express.
Finally, I would recommend you contact your local La Leche League group, or contact your local health department. Most health departments have a WIC program and should include a lactation consultant. I'm also including some links to La Leche League's website which might be helpful.
http://llli.org/NB/NBmilksupplyissues.html
http://llli.org/NB/NBpumping.html
Good luck!
J.