I have around 1.5M miles. I still have a lot of flying ahead of me though. The miles will absolutely devalue, but no one can predict when and by how much.
I would recommend using them when you can get a decent deal but don't just throw them away. Otherwise, you're basically devaluing it yourself.

Only you can determine the value.
For example, some people would say you should use miles to book J seats because their price is thousands of dollars. However, if you would never pay out of pocket for a J seat, then your value doesn't match its price so in my opinion, it would be a waste. I typically look at how much I think something should cost given what I usually buy and market conditions and then I compare it to it's price. Then I try to get better than approx 1.5cents/mi to decide if its worth it.