How does bulletproof glass respond when fired at by a 9mm at point blank range several times?
It depends on the glass and the bullet. Usually "bullet-proof" glass assumes the glass won't be struck in exactly the same place repeatedly. There are specifications for this that discuss things like the weight and speed of the projectile, etc.
One of the major bodies that maintains standards for this is the National Institute of Justice (NIJ). You can reference their document entitled "Ballistic Resistance of Personal Body Armor" at http://www.ojp.usdoj.gov/nij/pubs-sum/183651.htm. Definitions of the common "levels of protection" are on page 14 of the document.
To specifically answer your question, 9mm projectiles are mentioned in three of the NIJ standard Types or "levels". Type IIA discusses protection of 9mm 124 gr Full Metal Jacket (FMJ) bullets impacting at a minimum velocity of 1025 ft/s or less whereas NIJ level IIIA discusses High Velocity 9mm FMJ Round Nose bullets, 124 gr, impacting at a minimum velocity of 1400 ft/s.
When you purchase bulletproof glass, you typically purchase it with a specific level of protection. Glass that will stop 30-06 Armor Piercing rounds will be significantly thicker (and heavier) than glass certified to stop handgun rounds such as the 9mm.
As noted above, glass will eventually fail if struck repeatedly in the same location. It's just a matter of what you are hitting it with and how hard. Additionally, perpendicular hits will cause more damage than oblique hits that are deflected, thereby redirecting some of the energy rather than having 100% of it absorbed by the protective material.