At the end of the seventh year of the Sh’mitah cycle, all loans are nullified, including credit agreements and wage agreements that have been converted to loan agreements. (As the guidelines regarding rental agreements that have been converted to loan agreements are complex, it is best to make a Pruzbul in such cases.)
One who wishes to collect his loans after the Sh’mitah year must make a Pruzbul before the time that the loans are nullified (i.e. the end of the Sh’mitah year). Typically, this is done during the month of Elul.