a six-day period a nine-week period a seven-month period a two-year period a three-century period a fifty-million-year period As emsr2d2 demonstrated before, the hyphen is not included if it is not an adjective: She met him thirty seconds ago. We enjoyed five minutes together. I got to know him over the last six days. You have class for nine weeks. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.