18.75" same as they have been using for .38 Special for about 111 years.

Heavier (longer) bullets call for a faster twist.
But the 18.75" has been good for 158 and even 200 grain Specials for a long time now so there is nothing to worry about in 9mm

Most European 9mms have a 10" twist (probably 9.84", four turns per metre) and they don't "overstabilize" a 115 grain bullet, either.