I try to keep my camera changing to every other replacement, so its around a 4-5 years duration between each change. Not only would the improvements be more significant, it helps me to resist temptation from the latest models:bsmilie:
Use coolpix 4500 from 2003-2005, D70 from 2005-2008, D700 from 2008-2012 and still using it. Sold D70 at really cheap price and replaced with D5100 as 2nd body. Don't think will change D800 so soon as I have baby to feed now. Quickly upgrade when you are still single and when there is no commitment.