I am new to mean reversion trading, and I would like to get some good references about how to estimate the time it takes to a mean reverting process to cross its long term mean.
Well, "mean reversion trading" could mean a lot of things, I am not qualified to describe it in full generality.
However, there is a simple model of mean reversion called the Ornstein Uhlenbeck process that is often seen. It has two parameters \lambda and \sigma, where lambda is the strength of the mean reversion (so one over lambda is the mean reversion time).
Here is a nice web site that covers the estimation of OU process parameters from time series data using two methods: least squares (most intuitive) and max likelihood (more exact).
Hope this helps.