It is heavily dependent on drive speed and number of times you’d need to repeat. Each time you copy data into the RAID, the array would need to write the data plus figuring out the parity data; then, when you expand the array, the array would need to be rebuilt, which takes more time again.
My only tangentially relatable experience with something similar scale is with raid expansion for my RAID6 (so two parity here compared to one on yours) from 5x8TB using 20 out of 24TB to 8x8TB. These are shucked white label WD red equivalents, so 5k RPM 256Mb cache SATA drives. Since it was a direct expansion, I didn’t need to do multiple passes of shrinking and expanding etc., but the expansion itself I think took my server a couple of days to rebuild.
Someone else mentioned you could potentially move some data into the third drive and start with a larger initial chunk… I think that could help reduce the number of passes you’d need to do as well, may be worth considering.