Its done when you think its done. Simple as that.
More often than not, less is more. The first models I did I always tried to maks "showcase models". In fact most of the details I painted are not even visible at the gaming table. So I toned down the detail crazyness to a moderate level and my progress speed went up considerably.
Nowadays only single models get this "special treatment". On the table uniform and color composition is more important than painting every single microscopic gem.