Well, it needs to be as long as you need it to be. I think a mistake a lot of people make is trying to squeeze their keywords in there. Don't forget that your title is what shows in the Engines, so making it something that will get clicks is often more important than worrying about squeezing keywords in there!
Make a title in such a way that visitors will be forced to click on it. Putting keywords in it does will not affect the sites reputation. As melovemoney has said itself, that title is what shows in the Engines. If there are no keywords in your title then i don't think you will get any better results.