In Scribes, Script, and Books: The Book Arts from Antiquity to the Renaissance by Leila Avrin, on page 153, it is mentioned that copyists were paid average .25 denari per line (or 25 denari per 100 lines). How many lines did a good copyist write per day and how many characters were there on a line?! I'm thinking of time span around 200BC - 200AD.
‘Twenty-five lines an hour’ says a modern calligrapher who "worked with a quill to try to estimate the speed of a twelfth-century scribe."
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.