Traditional Tajwid explanations describe an appropriate ghunnah length as the measure of one alif. The point is to avoid both cutting the nasal resonance short and prolonging it excessively. It is also often taught as approximately two counts, although actual performance must follow the pace of recitation and correction by a teacher.
Example: «ثُمَّ» requires an audible, measured nasal resonance on its doubled mim; it is not discharged as an abrupt, ordinary m.
The khayshum (nasal passage) carries ghunnah, but the basic articulation of mim is at the lips, and that of nun involves the tongue. Nasalization does not mean abandoning the original consonant altogether.
In practice, verify the identity of the consonant first, then refine the quality and length of the nasal sound.