I don't know the exact answer to that, but "not much" in comparison to the time to send the bytes over the network. The actual contents are encrypted using AES which often has built-in instructions on modern processors and is thus very fast. The vast majority of the time is uploading time here.
Tahoe does use "convergent encryption" (basically, the key is based on the contents) so that the same file encrypted by the same client results in the same ciphertext (and thus, doesn't need to be re-uploaded).
I believe that only happens at the "capability" (i.e. file) level, though, not each chunk. So, if you had a directory of 10 files each 100MB and changed one, you'd only have to upload the new directory-descriptor and the one changed file -- but if you change a few bytes of a 1GB file, you'd have to upload all the ciphertext for that file again.
