We have performed a new and homogeneous analysis of all the Li data available
in the literature for main sequence stars (spectral-types from late F to K) in
open clusters. In the present paper we focus on a detailed investigation of MS
Li depletion and its time scales for stars in the 6350-5500 K effective
temperature range. For the first time, we were able to constrain the age at
which non-standard mixing processes, driving MS Li depletion, appear. We have
also shown that MS Li depletion is not a continuous process and cannot be
simply described by a t^(-alpha) law. We confirm that depletion becomes
ineffective beyond an age of 1-2 Gyr for the majority of the stars, leading to
a Li plateau at old ages. We compared the empirical scenario of Li as a
function of age with the predictions of three non-standard models. We found
that models including only gravity waves as main mixing process are not able to
fit the Li vs. age pattern and thus this kind of mixing can be excluded as the
predominant mechanism responsible for Li depletion. On the other hand, models
including slow mixing induced by rotation and angular momentum loss, and in
particular those including also diffusive processes not related to rotation,
can explain to some extent the empirical evidence. However, none of the
currently proposed models can fit the plateau at old ages.Comment: 20 pages, 10 figures A&A accepte