How long was a lunar month in ancient times?
'Day' is the worst possible unit to measure the revolution of the Moon around Earth as it is the temporal unit which is directly influenced by the tidal friction and deceleration. We can reasonably assume that the length of the year is not impacted by lunar tides and subject to much less variation.
This is only a partial answer as I will have to leave the math to get an actual number for time reasons.
The only measurable quantity for the past is the length-of-day of Earth and the tidal rythm by means of their impact on biorythms and in turn its impact on sediment stratigraphy and growth markers e.g. in fossil shells and similar.
One summary with some values and further references on how the past rotation can be estimated is this paper by Deines &amp; Williams. If you know the length-of-day at a time, you can calculate the yearly angular deceleration of Earth. That knowledge can be employed, using the conservation of angular momentum and calculate the orbit of the moon and thus length of month (thus one full orbit around Earth).