It can be as fast as an atmospheric switch flip in August and September. We see that even in mediocre years.
To your second question, it’s both, but the unfavorable MJO also promotes stability in addition to sinking air over the basin, which combined with it being the climatological peak of SAL, kills everything in the usual intraseaonal annual lull. The following images are from Michael Lowry. Note the massive spike of SAL in July with a rapid (albeit still high) decline. With the coming MJO change I’d expect SAL to be below the climatological norm by mid August even with additional SAL pulses.
I should also note that if we do see early August activity, especially if it’s higher end, that’s also likely a harbinger of things to come. Research shows that this period has actually been an annual nadir since the AMO flip in the 90s.