There are many synonyms for the phrase "eight years" that can be used for a variety of purposes in writing. A few possible options include "octennial," "octave," "octad," or simply "eight-year period." These synonyms can provide variation in language and help to avoid repetition. Additionally, using different synonyms can convey different tones or meanings - for example, "octad" may sound more formal, while "eight-year period" is more straightforward. Depending on the context of your writing, one synonym may be more appropriate than another. It's always wise to have a few alternatives in your writing toolbox!