The answer is always.. how long is a piece of string ?... there is no definitive answer.
I remember a friend of mine in the late 40s early 50s making a re appearnace at a London venue i frequented ,after 1 yrs absence.... my first question " where have you been hiding ? ".. I will never forget his reply.. he said " Ive been working on my feather and 3 with Charles ( Thieabault )...
I believe when students ask this question, its a relative one.. as good as ?.. fill in the blank .