My overall experience, running primarily Fixed Price listings for over a year now, is that about 60% of my buyers are purchasing in the middle of a 7-day run (3rd thru 5th day) - 10% buy in the first 24 hours - and 30% in the final 24 hours (with most of those being 12 or more hours before closing, rather than waiting til the last minute).
I am also finding that New items, more and more, don't sell on the first 7-day cycle - but do have Watchers from early on, who keep on watching through relist cycles, until the item finally starts selling in about the 3rd week.
I have also found, studying my detailed traffic stats, that 93% come from a specific eBay Search - 6% come from other search engines (Google, Yahoo, etc.) - and only 1% from just browsing around.
That knowledge, combined with a slow market in general, and my being stuck with a lot of slower moving stock, is what drives my plan to put EVERYTHING on the 30-day plan, along with auto renew til sold. And I'll see how it goes - being pretty confident it can't get any slower than it has been the past two months!