The phrase 'แค่ไหน (kâe nǎi)' is used to inquire about the degree or extent to which a certain condition or quality is available or applies. It can be used with adjectives and is often included in questions. When used in a sentence, it commonly follows the adjective or verb it is measuring. This grammar point is akin to asking 'how much' or 'how many' in English when referring to the intensity or degree of an adjective. It adds a level of detail and specificity to the question being asked.