How long to keep "notes" about tenant prospects

[Kenneth LaVoie]

My wife is our "leasing agent" and has these very organized notebooks where she takes notes about prospects during phone calls, and to take with her for showings. She just dumped 4 big notebooks on my desk starting in 2014 and asked me to "file them" if need be. How long might we hold onto something like this? I mean the only reason I can think of is that it creates a breadcrumb trail for actions in the event of a discrimination claim. Any thoughts, qualified or not, are welcome!