meters. But it depends.
Clay is finely ground rock, thats all. Where I live its boulder clay, formed by the ice sheet scraping the top ff the UK and dumping it in Suffolk over pre-existent chalk, as the glaciers retreated. Its a huge terminal moraine, and in some places its tens of meters thick, and in other places where the water ran different its pure sand and gravel, those being a coarse version of clay.
Up the top of a hill were I lived, it was about 4-6 feet into chalk: 10 miles west the chalk is at the surface, and they have a horse racing town. a few miles east is a gravel pit, and further in in a valley its clay deeper than who knows what.
There is no 'normal'