>Breeding parents 31/31/31/31/31/31 and 31/31/31/x/31/31
will produce a 31/31/31/31/31/31 Pokemon 1.8229% of the time or 1 out of every 54.8571 eggs!
So you will get a 6 IV offspring after hatching approximately 55 eggs. This is assuming that you have one 6 IV parent and one 5 IV parent, with one of them holding a Destiny Knot.
Here is a useful calculator for determining the chance of getting your ideal Pokémon.
However, a 6 IV Pokémon is not necessary to attempt to get unless you plan on running a mixed attacker, which is unreasonable for Mienshao.