The duration of a Lunar atmosphere?

If the Moon were to receive an atmosphere like the Earth, complete with oceans, how long before it returns to it's original airless dead condition?
I think that it would lose half of it's atmosphere after ten million years, the equivalent at sea level of an elevation of twelvle thousand feet, and all the rest after another ninety million years.
I am basing these figures on very quick estimates of the one percent of the Earth mass of the Moon - yes it really is that small - and the escape velocity.
Is that correct?