1) Like a seven year itch 2) Like the seven-year itch 
• (a.) Happening or returning once in every seven years; as, septennial elections in England. • (a.) Lasting or continuing seven years; as, septennial parliaments.
Sep·ten'ni·al adjective [ Latin septennium a period of seven years; septem seven + annus year. See Seven , and Annual .] 1. Lasting or continuing seven years; as, septennial parliaments. 2. Happening or returning once in every seven years;...
Septennial describes something lasting seven years.