The page title (btw, it's not a tag ;) ) can be as long as you like, however, from a search engine point of view, only the first 60-67 characters is the limit.

Although the length should be (as Engine says) less than 67 characters - I always recommend trying to put a concise, compelling and clear title of around 55 characters.

This forces you to cut out "wasteful" words and means you are always within all SE limits - plus, in my experience, a slightly shorter tag attracts more click-throughs - and that is the biggest benefit that a title "tag" can bring (apart from the obvious ranking boosts).

One or two phrases. Not more

As previously mentioned, the Title TAG is usually what will draw in a qualified visitor so it needs to be compelling. At the same time, it may require to be optimized for the search engine. How many characters exactly is determined by how many characters it takes. My rule of thumb is the shorter the better, especially if you are considering using the Title TAG to boost the value of a keyphrase. Avoid stop words (and, of, the ...) if possible as they take up real estate for nothing, usually.

Keep it concise and keep it attractive. Get the keyphrase in there and try to enforce it with a synonymous keyphrase, an abbreviation, acronym, antonyms, morphology, plural forms or applicable word variables. In this way your Title will appear to be intelligently crafted. The search engines like intelligent-looking stuff.

It depends on SE. Google read up to 70 characters in title while yahoo can read up to 64 characters. It depends on you which search engine you want to target for results.

I prefer 70 characters because most of the traffic come to my site are from google itself.