D.W.
There's no rule necessarily. We had bottles that my kids liked to have the slow flowing nipples on until they were close to a year.
If she's getting frustrated or seems to really be straining, I'd make a switch to the next level. The real difference is the flow (the containers will give you an estimated age).
I think we moved to level 2 nipples around 3-5 months and around 6-7 months went to mostly level 3 and 4 nipples.
Hope that helps.