Either way, to be productive, ie realistically writing production code, for a new platform in three days froms scratch, is a very big ask indeed.
I have 30+ years experience--perhaps that's why it doesn't take me long to get up to speed on something new.
Sadly, next year I will have had 40 years' at it. I finally started on ARM anout six months ago having moved from PICs, with 20 years on them, and their precursor CP1600 and LP8000 chips from GI in the mid 70s, with a large helping of Z80, 6502, 6800 and x86 in between.
I found with ARM I spent an awful lot of time working out the difference between the various cores, and then doing a shootout between some Cortex M4s from TI and NXP. I now use both depending on the application. But I'd say it took several weeks to get up to speed. Have you ever looked at just the clocking on the LPC4300 series? That's not trivial in itself, and that's just one small facet. You need to to learn their peripheral libraries, most of the underlying peripherals at a register and hardware level, the subtleties and features of their IDEs to name but a few. The same applies to TI, although they have the dicotomy of too much documentation, in that it's hard to know which document to start with there's so much of it.
If you can port an RTOS from ARM to PIC32 in a week, mastered the IDE and debugging tools, and can present it as a finished product then well done. But don't forget, there's a lot more to a system than just that RTOS. You can spend an entire day just getting your head round DMA, or a serial peripheral for example.