This change has been a long time coming.
So what does this sentence really mean — the change has taken a long period of time to finally occur? Or something different?
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This is a special idiom in the English language. It doesn't strictly mean what it says. Its meaning is really:
The emphasis is that it should have happened, or it was needed a long time ago, and now we are finally getting around to doing it.
For example, when the government passed laws to regulate the banks, some might say the change had been a long time coming because, in their opinion, the banks had been abusive of their unregulated status, and so finally that wrong had been righted.
Regardless of what you think of banking regulation, that would be the meaning. Not that it took a long time to happen, but rather than it had been needed for a long time.