How much breast milk you can pump will depend on your individual circumstances: when you're pumping, how old your baby is, whether you're pumping in place of a feeding (such as when you are separated while at work), or whether you are pumping in addition to feeding your baby full-time. It can also have a great deal to do with the pump you are using and how you're using it.
In general, if you are only getting drops, or a very small amount of milk while pumping, but your breasts still feel heavy and full after you've pumped for 10 to 15 minutes, then it is very likely that you are having difficulty letting down in response to your pump. In this case, it might help to experiment with different speeds (if your pump has this option). If you do this, it's important to use a setting that's comfortable for you. More suction does not mean more milk. In fact, suction that's so high that you experience pinching or pain can actually decrease milk flow. Making sure you're using a flange size that allows your nipple room to move can also help significantly.
On the other hand, if your milk flows well for a short amount of time, and your breasts feel soft and drained after pumping, then it may be an overall milk supply issue. There are many ways to increase milk supply, and it might be helpful to work with a lactation consultant.
Make sure you have a realistic idea of how much milk expression is normal in different circumstances. It can be very common for moms who add a pumping to feeding their baby full-time to only get .5 to 1.5 ounces of milk at this pumping session. Moms who are pumping in place of a feeding (because they are separated from their baby) can often pump three to four ounces. Both of these amounts are dependent on the age of your baby, the time of day, and the time since you last breastfed or pumped.
In general, it takes a little while to get used to pumping and, as you gain more experience, it will go much more smoothly.