The years of history the book covers are 445-431 B.C. or perhaps a few years after that.
In 445 B.C. (the twentieth year of Artaxerxes' reign, 1:1) Nehemiah learned of the conditions in Jerusalem that led him to request permission to return to Judah (2:5). He arrived in Jerusalem in 444 B.C. and within 52 days had completed the rebuilding of the city walls (6:15).
In 432 B.C. Nehemiah returned to Artaxerxes (13:6). He came back to Jerusalem after that, probably in a year or so. The record of his reforms following that return is in the last chapter of this book. Apparently Nehemiah completed all of them in just a few weeks or months.
Even though the book spans about 15 years, most of the activity Nehemiah recorded took place in 445-444 B.C. (chs. 1-12) and in 432-431 B.C. (ch. 13).