In the present work, we provide the exact answer to the title question employing a probabilistic approach. The average number of Langmuirs < L > required for monolayer formation was found to be equal to Sigma(i=1)(n) (1/i), i.e., the armonic series up to the nth term, where n is the number of adsorption sites. This result is particularly useful when a reduced number of adsorption sites is considered, such as adsorption on small terraces of nanoscopic dimensions where the value of n could be in the range of a few thousands sites. In this case, the use of integrated equations derived from the mean-field approach would provide completely misleading results.