I would say that it really depends on the age and appetite of your baby and even the time of day. It also changes from woman to woman, based on your storage capacity. (How much room you have to store milk between a feed) Women with a smaller or larger storage capacity can make the same amount of milk, it's just that they will have more or less milk at each feeding/pumping session. A women with less storage capacity just ends up feeding more times in a day compared to a woman with a larger capacity. Your current supply is also based on the current demand. Also pumps may not empty the breasts as well as a baby. To help your breast empty, hand expressing into the bottle, after pumping can be a really good thing to do.
Were you asking for any particular reason?
With that said, I remember when my baby was a few months old, I would pump until the flow stopped and I think I would get around 4 ounces, sometimes less. I really don't know how many minutes it was. As he got older, it was closer to 8 ounces. ...and then after 6 months a slight decrease can naturally happen. But again, there are just so many variables in play there.
Please feel free to let me know if you have any questions about anything I've mentioned! ...and great job you for breastfeeding.
4 oz in one pumping session is on the high side of average. I think Kellymom says that an average pumping session is 1/2 to 2 oz total.
For a date night, just figure that your DC will eat an average of 1 to 1 1/2 oz per hour that you are gone. Since it's not too much quantity, and since defrosted breastmilk is only good for about 24 hours, I would recommend freezing in 2 or 3 oz bags so you only need to defrost exactly what you need.
If you suspect oversupply, but it's causing no problems, then I wouldn't worry. I had a slight oversupply for the first few months, but noticed it level off at about 6 months. I pump at work and went from 5 oz per pumping session to 3 oz each time, but my supply is still good. Just FYI so if it happens to you, you don't interpret this as your supply drying up.