Rubato is only one way of playing with time.Aside from the fact that compositions can have changes in time signature (sometimes frequent), there are fermatas, accellerandos, ritardendos, pauses and temporary tempo changes that also come into play.First step is to look at the score, then see what he is doing with it. Then play around and use your ears.
I never liked the idea of "giving and taking time" I think it's heavily inaccurate and a far too mechanical way of thinking of rubato. No, your Chopin nocturne should not follow a metronome. No, when you take time, you do not need to speed up in order to make up for it. All you have to do is return to the original tempo. Thus, you take time, and never really give it back.