Hyphens, En Dashes, Em Dashes

Q. How should the following be hyphenated, if at all? Two and a half hours.

A. There is no need for hyphens if you’re using the phrase as a noun: We’ll be there in two and a half hours; two and a half hours is plenty of time. If you are using a phrase like that as a modifier, however, you’ll need hyphens to hold it all together: a two-and-a-half-hour trip.