Those are good questions.
Like any beer, the time it takes to reach a stable FG depends on several factors: wort composition, Brett strain(s), temperature, etc.
It does take longer than Saccharomyces to finish eating whatever dextrins it wants. Typically it takes at least 3-4 weeks, and perhaps up to 6-9 months.
I haven't been monitoring gravity on my traditional sours, so I can't really speak from experience to say when mine actually finish.
The lengthy aging is mainly for flavor development. Generally the fruity flavors transition into more barnyard-type funky flavors over time. Blending can achieve a mix of both.
Depending on the hop rate, souring may take a very long time, up to 18-24 months in some cases.
FYI: With more modern pitching schedules and wort production techniques we can get mixed Sacc/Brett/Lacto sours bottled in the same amount of time as a clean beer (1-2 weeks) if you really want to push the timeframe.