depends on what you are using it for, DS was prem an I started for 6 weeks on a hospital grade pump, pumping 6ish times a day. At that stage I would get 6-8 oz first thing in the morning, 3-4 oz at 11pm and overnight.
I then switched and bought a double medela freestyle pump, which yielded the same as a hospital garde pump. I still used the hospital pump when in hospital and can genuinely say at the time, the yield was the same. Varied on time of day but no more.
Still using the freestyle medela pump, 8 months after ds was discharged, now I am back at work. I regularly get 4oz when expressing now, rarely more but that's because ds is older and doesn't take as much (he will never take more than 4oz in a bottle when I am away from him- he used to happily take 7-8 oz in one go).
I find the electric pump a godsend as I can work, type, read etc while expressing and I have basically pumped almost daily for 10 months for variety of reasons over the months from I want to sleep, drink, work, go out to i am full and uncomfroatble. Because i pumped all the time I founf having my hands a godsend, and made expressing a lot less hassle.
I pump for 15 ish minutes a session, always had. the general recommendation is not to pump for more than 15-20 and if you want to increase supply put another short session in rather than pump for longer.