User blog comment:Juraquille Mihawk/Jewelry Bonney and Gorousei/@comment-3242083-20110312180421

I had the same idea a few months ago, I think it is like you said, or Bonney used her powers on a high ranking marine (who wants his old form back, badly).

I believe that Bonney's powers last until she looses consciousness, dies or reverts it by herself. Remember, we already know of confirmed fruits that seemingly last "forever":

Shiki's Fuwa Fuwa no Mi - Able to levitate objects once touched, lasts till the user looses consciousness.

Decken's Mato Mato no Mi - links the ability to the person he touches, seems to last at least 10 years, likely forever since he calls it a "curse"