In 1 decade there is 2.5 Olympiad.

century day decade dog watch dog year fortnight helek hour [hr] indiction jiffy jiffy [alternate] jubilee [Biblical] lustrum millennium millisecond minute [min] moment month [28 days] month [29 days] month [30 days] month [31 days] Olympiad quarter [90 days] score second [s] week year [365 days] year [366 days, leap year] year [Gregorian] year [tropical year]

century day decade dog watch dog year fortnight helek hour [hr] indiction jiffy jiffy [alternate] jubilee [Biblical] lustrum millennium millisecond minute [min] moment month [28 days] month [29 days] month [30 days] month [31 days] Olympiad quarter [90 days] score second [s] week year [365 days] year [366 days, leap year] year [Gregorian] year [tropical year]