Gradually, because every day opportunities are missed, little bits of value are lost, customers become unentranced. We don't notice so much, because hey, there's a profit. Profit covers many sins. Of course, one day, once the foundation is rotted and the support is gone, so is the profit. Suddenly, apparently quite suddenly, it all falls apart.
It didn't happen suddenly, you just noticed it suddenly.
The flipside works the same way. Trust is earned, value is delivered, concepts are learned. Day by day we improve and build an asset, but none of it seems to be paying off. Until one day, quite suddenly, we become the ten-year overnight success.
This is the way it works, but we too often make the mistake of focusing on the 'suddenly' part. The media writes about suddenly, we notice suddenly, we talk about suddenly.
That doesn't mean that gradually isn't important. In fact, it's the only part you can actually do something about.