I nursed both of my boys until they were 10 months, and 14 months. The smell of formula made me gag, so I only used it if I absolutely had to. What I found was that the more I nursed, the more I produced. I kept up my own liquid intake, and paid close attention to how what I ate affected my milk. My boys didn't like anything carbonated, with caffeine, or garlic and onions. Oh yeah, chocolate didn't help either. My youngest son would get a red rash on his face when I ate barbecue sauce! Now it's his favorite condiment!
Check with your doc for amounts if you keep up with formula. I really made up my mind that was going to nurse, even with some very difficult challenges at the beginning that almost made me give up. My positive mindset, determination, and focus are what got us through
As far as frequency, I tried to space feedings initially at atleast every two hours when they were so tiny, the older they got, the longer the spread. I remember being exhausted, but you find a way to get through. I also learned to put my babies down for sleep while they were still awake, so they would learn to soothe themselves. If I put them down asleeep, they would wake up and pitch a fit.
Good Luck! These times pass way too quickly, so don't hurry them.