When there is a quantity, measurement, distance, value, amount that is expressed as a direct adjective (right in front of a noun) AND there is an “s” on the adjective, use an apostrophe “s” when it is singular and an “s” apostrophe when it is plural.
…one minute’s delay
…five minutes’ delay
…one week’s vacation
…two weeks’ vacation
When there is no “s,” use a hyphen.
When there is an adjective rather than a noun after the combination, there is no apostrophe or hyphen.
…seven months pregnant
…one month long