Breastfeeding Constantly

My baby girl is 18 days old. My milk took a little longer to come in so my pediatrician advised me to supplement formula after feedings at my 1st checkup to get her back up to birth weight. She loved it and seemed super satisfied. She put weight back on right away. Now I've been trying to get to exclusively breast milk but she only seems satisfied about half of our feedings. I know my milk production is not an issue because when I pump there is more than enough. And starting yesterday she seriously wants to nurse for hours. She will cry start rooting around wanting to suck on everything as soon as I pull her off the breast even if she falls asleep, the minute I put her down its the same thing. The only thing that seems to satisfy her is formula and even then sometimes she wants back on the breast. I know newborns are growing and need to eat a lot but is nursing 2-3 hours normal? I can't ever get anything else done!