How often to pump?

Hey.. so my baby boy was born at 33 weeks, 3days and the doctor wants me to breastfeed. He's tiny, so he's having latching difficulties, so instead i decided to pump. A couple of questions i have is how often do you pump and how long do you pump in each session? Is it okay if i pump for about 15mins for each breast, take a break; then pump again? Im trying to increase my supply because the hospital keep contacting for more milk. Whenever he runs out, the use neosure instead, but i want him to get as much breast milk as possible. Any suggestions on how to increase my supply also. Please and Thank you☺