Don't know, but fairly high.
Eventually, I imagine, nylonase will bind to insoluble nylon and hydrolyse it on the fiber. The difficulty in hydrolysing insoluble polymers is the primary reason cellulases are rather rare.
According to this site, the number of monomer units in nylon 66 is 100 to 250.
More interesting stuff on this site.
Cheers!
...and Merry Christmas!