Looking for experiences changing status from J-1 au pair to B-2

I'm wondering about the process when someone with a J-1 au pair visa requests a status change to a B-2 tourist visa. When they file I-539, they request a start date for the B-2. What happens if the USCIS takes a while to rule on it, and the requested date for the status change passes? Is this likely to result a shorter amount of time granted for tourist status?

Also, how long of a B-2 stay is usually granted for someone who has worked as an au pair and is ending the program in good standing with their sponsor and host family? I've heard 6 months is fairly common, but I'd appreciate some more input. Thanks!