You really don't need to switch to a higher flow until it's obvious that SHE is getting frustrated with the slow flow. It's good for her mouth anyway- babies need to suck, and it develops the muscles needed to eat and talk. You could try alternating between a slow flow for one feeding and a medium for the next. After a day you will probably be able to guess her preference.
My son used the newborn nipples until around 9 months. I switched him to medium flow, and around 10 or 11 months I started to introduce a sippy cup. He took the sippy cup well, and looking back I probably could have just started the sippy cup at 9 months instead of spending money on nipples.
When she is sucking harder than it will come out most of the time, and getting frustrated, I would switch her. It will be messy for a week or two, but it wont take long for her to adapt and learn to control the flow.
Also, if she continues with the newborn nipples, keep an eye on the quality of the silicone. After a while they start to break down just a little bit. A few of ours started to turn white, and sometimes they can rip. Most nipples say on the package when they need to be replaced. I used Avent and they suggested replacing them after 3 months...but I think that is probably another marketing tool to get you to spend more money. I just inspected them after washing and only a couple seemed to be worn.