The noun form is open (a half hour) The adjective form is hyphenated (a half-hour session) For simple fractions: Hyphenated in noun, adjective, and adverb forms, except when second element is already hyphenated (one-half; one and three-quarters; one twenty-fifth) So "1/2" should always be written out as one-half. (Unless it's in a sentence like ... 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.