Prof (Terence) Wade ('A Comprehensive Russian Grammar') agrees with Vovan! He describes this по + acc usage as a "distributive" sense, i.e. not modifying the meaning of the quantified number of days, and there are many examples of it on Google:
§448 по + dative/accusative in distributive meaning (...) Note The accusative, formerly confined mainly to colloquial styles, is now freely used in written styles also: (...)
Рабoтают по семь часoв в день. They work seven hours a day.