"Sounds like it worked for you, but for ther record....
Block checking can take an incredibly long time if you have actual bad blocks that cause I/O errors; depending on the driver, the error and what it uses to recover. SSDs should be faster and presumably less likely to have errors, but errors could still make it many times the best-case scenario.
I'd suggest using dmesg to see if it is having problems if it seems overly slow.
BTW, why do you think it must write to the whole disk? I don't see from the mkfs.vfat(1) man page where it does a destructive test, the badblocks(1) program does a read-only test by default.
And FWIW, I've had mkfs -c take 24+ hours (IIRC) on a SCSI disk that was ~4GB."
