Originally Posted by cocohonk
Ah, grasshopper, I might ask in return: how long is a piece of string?
Both questions are relative. There is no absolute answer.
Sustain in general means a note continues to reverbrate within the instrument and can be heard after the note is plucked. But that is merely one part of the equation when assessing good sound. The harmonics, the volume, the attack, the warmth, the fullness... what sounds best to your ears can't be measured except by that little technician inside your own head.
Overall, notes that last longer are better because they provide more complex harmonics, therefore a richer sound. But too long and the sounds get muddy. Play and hear. Does it sound good to your ears? Rich, full, yet clear? If so, that's good sustain.