I'm not sure there is a magic length of time; I think mochtegernvrtuoso's idea is that it is better to set out to do a little practice regularly, than aim for a 3-hour scales marathon and become frustrated. Everyone needs to define their own version of "little".
Raspberry, I get your point that you want to have serious goals, and therefore like exams. However, I'm not sure that exams alone are giving you what you need. If they were, you wouldn't be considering giving up. There are a million different equally-serious goals you could set yourself(*). For example, if there is a piece you love and would like to play, then it could be a goal. Maybe that's something to discuss with your teacher? In a way, musical goals such as much-loved pieces, or the exploration of the repertoire of a composer you admire, are longer-lasting than exams, because eventually you will run out of exams to take. And anyway, I get the impression the three motivating factors that allow someone to pursue music beyond the end of their exam career are various personally-determined combinations of the satisfaction of teaching, the satisfaction of performing music that one loves, and cash! So it's no bad thing to think about the music you love, and what you want to do about it.
(* goals aren't exclusive. You can, of course, do exams as well as having other goals such as learning a great piece. And there are so many other goals too: to be able to accompany a friend, to be able to play nursery rhymes for a grand-daughter, to have the confidence to play your last exam piece on one of those public pianos in a shopping arcade/station (that takes serious bravery!)... and probably more).
(edit: embarrassing correction of there>their)