Don't forget it's also by slot. Some slots (such as weapons) will find upgrades frequently. Other slots you may find yourself using the same level 70 item into level 80 raids.

With BC I switched my main to my mage, and only cleared BWL with it, yet still had two items last me all the way to 70 (Grand Marshal's Spellblade and the caster dps pants from Ossirian in AQ20.) Half my gear lasted till the mid 60's. I expect Wrath to be the same, getting a few upgrades from T4/T5 right away but still using a few pieces till close to 80.