The floating m doesn’t happen immediately, but the color dissolving off of the M&M happened almost immediately. The materials used to color the M&M start to dissolve quickly and you begin to see the colored dyes from the M&Ms dissolve and make rainbow colored water!
The chocolate on the other hand did not dissolve quickly, but we wanted to see the floating m!
It took about 10 minutes for the first M and about another 5 or 10 minutes for all the M’s to float. Using warmer water will make the M float faster.
And there it is! The floating m! Why does the m float? It’s made out of edible paper. You can see this paper on cakes and looking into the cup you can see the M is on a light square paper.