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βTwenty-five lines an hourβ says a modern calligrapher who "worked with a quill to try to estimate the speed of a twelfth-century scribe."
See CLEVER SLUGGARDS? HOW FAST DID MEDIEVAL SCRIBES WORK?
The article analyzes the evidence for midieval scribal speed, with detailed references for your further study. The calligrapher's experimental result of 25 lines per hour is probably slow for a professional scribe, but gives a starting estimate.
The article also notes that lettering styles were more complex/difficult in late antiquity, and so would have required more effort per line of text -- that is, writing would have been slower.