How long?

You could have eaten them even a year later. As long as they still look and smell normal, and don't fail the taste test, you can still eat them well past their best-before date.

The best before date only indicates the date up to which the manufacturer guarantees that the product is fine, but from this point onwards the products are definitely not certain to be fatal. There used to be no best before date at all; it was only introduced a few decades ago. My parents, even at 60 and 65, still don't have the best before date because it was only introduced afterwards (to be precise, it has only been compulsory since 1981). Back then, people relied on sight, smell and taste and didn't die from eating biscuits that had been lying around for a year and were most likely only good for a few weeks according to today's best before date standards.

By the way, I'm not at all concerned with the best-before date. I rely on sight, smell, and taste, and as long as things don't fail those criteria, I'll still use things long after their best-before date, even several years after their best-before date. And without any health problems when consumed.