Pretty often, I think. I think that in a large fraction of kingdoms, there's a roughly dominant strategy, and if one person does it and the other does not, that'll decide the game right there. However, most such matches aren't boring - even if both players have the same general idea, the devil is in the details. Sometimes it's worth it to grab slightly too many of a card to deny some to your opponent, sometimes you need to decide when to green based on what your opponent is doing, you can pick up engine components in a better order than your opponent, and so forth. There's a lot of things that add up to 'playing better' even if you've already decided on an overall strategy.
If you mean that you're going to buy the exact same things all game regardless of what your opponent is doing, probably never. At the very least, considerations like PPR will come up in every close game, even BM+Envoy.
...I guess it depends in the end on how narrowly you define "strategy X". Like, in a lot of games with, say, Goons, the optimal strategy is "an engine with Goons", but there's a lot of variation in how you can put together that engine, and with goons engines you basically always have to respond to your opponent based on how low piles are.