From The Paper of Montgomery County (Crawfordsville):
Q: How long should the newspaper keep information on public notice advertisements that have been placed by local entities?
A: Three years should be sufficient to keep such records.
The statute of limitations on most legal actions runs out in two years, so you would have the documentation to show when items were published during that key time period.
Contact Steve Key, HSPA executive director and general counsel, with media law questions at email@example.com or (317) 624-4427.