Thirty days was the usual time of mourning for persons of high place and eminency. See Genesis 1:3,10 Num 20:29. For others seven days sufficed.
Related Commentaries of Deuteronomy 34:8
Deuteronomy 34:8
8 And the children of Israel wept for Moses in the plains of Moab thirty days: so the days of weeping and mourning for Moses were ended.