do you mean, the time needed in vacuum in order to form a monolayer ?
If that is the question, the answer is that it all depends on how good/bad vacuum is. In my experience (long time ago) I once calculated that in order to deposit a very clean film of Germanium film, not contaminated by residual gas physisortion/chemisorption, I should keep deposition time under some minutes, because a monolayer of "dirt" could form in a few minutes when the vacuum was of the order of 10^-11 torr (ultra high vacuum conditions).