This is incorrect.
BFA - Pre-patch to PTR June 18, Expansion release August 14 - 8 weeks of PTR
Legion - Pre-patch to PTR June 18, Expansion release August 30 - 10.5 weeks of PTR
WoD - Pre-Patch to PTR September 15, Expansion release November 13 - 8 weeks of PTR
It isn't a month of pre-patch PTR - it's consistently around 2 months, and then a month of pre-patch before the expansion release. October is all but out of the question, especially since they haven't announced a release date, and announced a date less than 3 months out for each of the previous 2 expacs. We are looking at around mid-November.
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Pre-patch PTR has been 8+ weeks for each of the last 3 expansions. You literally have to go 4 expansions and 8 years in the past to find the last time the pre-patch PTR lasted less than 8 weeks (5 weeks in MoP).