Rule of thumb for decanting aged wine?

So I’m newer to fine wine and am beginning an aged left bank Bordeaux kick. On a website, I’m buying 1990-2010 Bordeaux, often spending $40-70 per bottle. I’ve heard that with some older wines, letting them decant too long is a bad idea as they’ve already lost some flavor and you should drink them right away? Others clearly need a 1-3 hour decant.

So the question is, what’s the rule of thumb on decanting? How do I know if a bottle should be drunk immediately or decanted? Is there a particular age or variety where decanting is not a good idea? On the flip side, is there a particular age or variety where a very long 2-3 hour decant is recommended?