The problem with leading and Glock bores isn't that it's certain to happen or that it always progresses rapidly, but rather that it's impossible to predict how rapidly it will progress and so there isn't any good rule of thumb about what you can and can't get away with.
One book (The Glock in Competition) records the results of testing using pressure measuring equipment that showed that "identical" Glock pistols shooting identical ammunition leaded dramatically different amounts resulting in dramatically different discharges. One G30 after 75 rounds showed TWICE the pressure increase due to leading as another "identical" G30 using the same ammunition showed after 300 rounds.
Basically, the only person (a co-author of the book referenced above) I know of who has done actual pressure measurements on Glock barrels using lead bullets recommends against shooting lead bullets in Glock barrels and stopped doing so himself.
Here are some other posts I have made on this topic.