I would say that you can't say when they do change their liveries... It always depends on the situation the airlines are in. I'll take Qantas and Air France who both have had their liveries for a long time now but they still look "up-to-date" (which doesn't mean that everybody likes them...) so they probably don't seem to see the need for a change or take Delta Air Lines, they had one livery for 30 years or so and then decided back in 1997 to change it and two years later they again decided to change it to DeltaFlot as it is today. So it can happen that we will see a new Delta livery in a year or two or in 30 years...
But if a certain livery gives a bad image to the airline because of relating to a chrash or so then sometimes airlines use the tool of changing it too.
US Air is the best exampel. After they called themselves US Airways and changed the livery nobody remembered the chrashes of the 737's (neither did the 737 fleet since none chrashed after that) and their image was good from then on... And I love their A330's!!! (FRA-PIT and PHL-FRA back in 2000 and 2001)