(re-sending because I wasn't on nanog-post)
For example if we change ip addresses will we need to plan on 20% traffic at old site on day1, 10% day2, 5%, day3, and so on...? There are also issues related to proxy servers and browser caching that are independent of DNS we will need to quantify to understand full risk. The more data we have will drive some of our decisions.
You might consider the following paper from IMC 2003: "On the Responsiveness of DNS-based Network Control" by Jeffrey Pang, Aditya Akella, Anees Shaikh, Balachander Krishnamurthy, Srinivasan Seshan, http://www.imconf.net/imc-2004/papers/p21-pang.pdf It sheds some light on how widely DNS TTLs are adhered to. The CDF graphs on the 4th page suggest that you should be fairly safe after a day, though I don't see if the paper specifically states what the largest recorded violation was. Sharad.