When is a long tail heading , to long. Or taggs to long. Often my headings are about 15-20 words. With 10 keywords in the tag. Im curious to hear about the boundaries.
and the keyword tag is about 20 keywords. That's the rules that been used in many keyword density analyzer.
As the old saying goes, the shorter, the better (in most cases anyway - too much of a good thing can be bad for you). Worry more about writing titles and copy that are relevant to your readers and are easy for them to read and understand than writing for the search engines. Afterall, optimizing a site or page for the SERPs won't do any good for you if the people who find the page or site in the SERPs can't understand what you're talking about.