That's 4–6 months in the 18 months the trials lasted for, i.e. about a 30% slowdown of progression. The open-label extensions suggest this relative slowdown seems to continue at least to the 4-year mark (at which point it would have bought you over a year of time): https://www.alzforum.org/news/conference-coverage/signs-last...

Time will tell if the 30% slowdown continues beyond four years, and/or if earlier treatment with more effective amyloid clearance from newer drugs has greater effects. The science suggests it should.