How long have you known challenges you on compound progressive tenses.
English verbs have a wide and versatile array of tenses and 'pseudo-tenses' to convey quite precise shades of meaning: when within a time sequence something happened, whether it is/was an ongoing continuous action, and whether it is/was open-ended at either end or both (e.g. until when, as from when). This Quiz offers you the chance to refresh your awareness of some of these subtleties.
'How long have you known?' is the poignant question that could relate to such circumstances as a pregnancy, a potentially fatal degenerative illness or some other intimate secret. In some other languages the structure would be more like 'Since when do you know / are you knowing?' (Not good English, though understandable enough, particularly in important circumstances.)