18 months is the current maximum processing time posted on the IRCC website. I guess December will be the deadline for this cohort.
I am sure there is no max / cap on processing time. The 18 months is the (current) AVERAGE processing time (for 80% of applications). If the average processing time is exceeded, then can apply for an Order of Mandamus, where a federal court hears requests and pressures IRCC to decide on the application. Mind you, you will have to hire a lawyer and meet certain conditions.