The Pratomagno Alps divide the province of Florence and Arezzo and separate the Casentino valley from the Val di Sieve on the Florence side.
By car it takes about an hour and a half to reach either Vallombrosa or Reggello on the Florence side...
I assume you are looking to hike up? Reggello is probably the best starting point as there are many marked paths (red and white signs by the CAI association) that go up to the "Croce di Pratomagno", the highest point at 1592 meters a.s.l. but there are also paths that start from Pian di Scò. You can also just reach Monte Acuto (1131 m.a.s.l.) for a shorter hike.
From Vallombrosa, paths take you to Monte Secchieta (1449 a.s.l.), also part of the Pratomagno range, and from there you can go along the crest to the Croce di Pratomagno. The whole range has paths on both side (Florence and Arezzo) but from our map it looks like the Florence side around Reggello has the most options.
I hope I've answered your question, and please let me know if we can be of further help.
The top of the Pratomagno range starts about 40 km almost due east of Florence, then they extend southward for about 15-20km.
The basic coordinates as given by wikipedia in Italian are: 43°39′N 11°39′E
(Pratomagno - Wikipedia)
The drive takes 90 minutes because I was calculating taking the more common 2 lane roads (SS67) to Vallombrosa or Reggello, but if you map it on Google (Florence-Reggello), the directions given make you take the Autostrada A1, with an estimated time of around 40 minutes. So depending on how you choose to get there, the time will change.
Hope this information makes it clearer!