I think you can only use 63 letters in a title tag. You won't be penalised but your title will become broken in SE results.
15 words are too long. As MeetHere said title will become broken in SE results if it is too long. Try to be specific in using a title, omit stop words. If necessary use | for replacement of "and" and "the" words.
The idea is to make them as short as possible while still being descriptive and containing your keywords. If possible make them exactly what you would expect a user to type in to search to find your page. For example if your page is about explaining SEO to a beginner, i would make my title "What is SEO" It's short, descriptive and most probably what someone would search if they were looking for that info.
That's why you try and match exactly what the user types, higher rankings and higher click-thru because it says exactly what they are looking for.
I recommend max 63 characters, and in those characters try to include your keywords targeted for SERP`s
What Zippo said. Also make sure to tag your biggest anchor texts early in the results. Internet Marketing - Site.com instead of Site.com - Internet marketing.
exactly as they have all said, your title will become cut off, but if you structure it right, it can be cut off at a point where it wont look disjointed
IMHO Google heavily discounts words further in the title. The first few words are worth a lot, and the rest are almost ignored. There is no sense in having 15 word title except for fooling SEs. Not many users will have a screen wide enough to display the whole page title. Firefox bookmarks only show 5-6 word titles. I'd have 5-6 words in a title (and wouldn't put the domain name there).
search engines will not ban you because of 15 words title but they will not send visitors to that page too. i think titles must be 50-100 chrac.
Your title tags are arguably your most important on page factor. Some may debate this but your splitting hairs, let's say it is very important. Try for your top three keywords for that page with the most important first etc.. And remember, when your ppage shows up in the SERPS, click through rates will be strongly affected by your skill in constructing your title.