Set Bonus VS iLevel & Raw Stats [closed]

When you have 2 or 4 (or all) of the Set Pieces of a Tier of PvE Gear, you get a Set Bonus, which could include some spells doing more damage, or having their cooldowns reset, etc.

My question:

Is there an average or standard/generic case of when the Set Bonus is inferior to a higher item level or other raw stats?

For instance, when you have 4/5 Set Pieces, and you pick up a new Chest piece, 8 Item Levels higher than the Chest piece from the Tier Set you currently have on, should you replace it?