the answer depends on sonication intensity and the degree of change in aspect ratio you consider as 'lose their L/D'. It will also depend on the MWCNT in question.
The change in aspect ratio after 20 min sonication will be usually very small.
It depends on the frequency of the sonication and on the concentration of your MWCNT; if you have a high concentration with respect to the aspect ratio, you'll probably have formation of aggregates and the interaction between the CNTs will be high and maybe they won't break as fast as if you had a lower concentration.