Here is my method:
Using MileSplit's database of 2018 outdoor track times for the mile race and converted 1600m races, I found that the mean girls' time was 4:41.38 and the boys' time was 4:04.52.
The difference between these two means is 36.86 seconds, and the ratio is approximately 1:1.15.
These data give us a rough idea of how girls' and boys' times compare in the mile and 1600m in the most recent full outdoor season. We can now use this to figure out roughly what the girls' equivalent is of a 5-minute-mile.
If we use strictly the flat difference, the output guess we get is approximately 5:37. If we use only the ratio, we get approximately 5:45. Based on these results, I conclude that an appropriate round number for the girls' equivalent of a five-minute mile is 5:40.
Keep in mind that my above methods are far from scientific, and therefore severely lack in both precision and accuracy. This is not a definitive answer.
I hoped that helped to answer your question!