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From the wikipedia entry on "anniversary"

semiannual: 1/2 year
annual: 1 year
biennial: 2 year
triennial: 3 year
quadrennial: 4 year
quinquennial - 5 years
sexennial - 6 years
septennial - 7 years
octennial - 8 years
novennial - 9 years
decennial - 10 years
vicennial - 20 years
semicentennial - 50 years
demisesquicentennial - 75 years
centennial - 100 years
quasquicentennial - 125 years
sesquicentennial - 150 years
terquasquicentennial - 175 years
bicentennial - 200 years
tercentennial or tricentennial - 300 years
quadricentennial - 400 years
quincentennial - 500 years
sexacentennial - 600 years
septuacentennial - 700 years
octocentennial - 800 years
nonacentennial - 900 years
millennial - 1000 years
bimillennial - 2000 years