Although relatively new to BB (2nd Season), I was under the impression that between 55-70 minutes was optimal for GS pop. Now, I've been monitoring my players closely and this seems like a reliable range. I agree with Hadron that it is most probably a cumulative process.
Additionally, training GS when minutes have been poorly managed for the week doesn't necessarily prevent a player from having his GS improve. My new centre, newly acquired, had played 117 minutes prior to my purchasing him. Due to some significant restructuring of my roster, I had to train GS to limit his potential for injury this week. So far so good. But my point is this: improving the GS of a player is, for me, a hit and miss process in general. Keeping my players minutes within that 55-70 range has shown the best results for me thus far.
Matty