An article talking about the timeframe for this or that sent Mr. Rewrite diving for his reference books recently.
The AP Stylebook doesn't address this one, so Mr. Rewrite dutifully followed AP procedure and sought guidance from Webster's New World College Dictionary. It lists time frame. That's good enough for Mr. Rewrite. He'll go with it.
It's a good idea to pick a style and stick with it, as the media are all over the place on this one. Mr. Rewrite found major news organizations, including AP using timeframe and time-frame rather than time frame.