How many letters does the top answer for “of or relating to or completing a period of 100 years” have?

The most common answer is centenary (9 letters).

Crossword clue: of or relating to or completing a period of 100 years

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

centenary 9 letters wn-def

of or relating to or completing a period of 100 years

centennial 10 letters wn-def

the 100th anniversary (or the celebration of it)

of or relating to or completing a period of 200 years 100% similar → bicentenary, bicentennial

of or relating to or completing a period of 300 years 100% similar → tricentennial

a period of 10 years 50% similar → decade, decennary, decennium

a period of 100 years 50% similar → century