Is there a correlation between depth of concentration and the length of a practice session?

I am asking this because now, after three years of daily samatha, I sit between forty-five minutes to an hour. Should I be sitting longer? I stopped timing myself about two years into practice, and generally speaking I sit until I emerge spontaneously from a state of concentration. Unless I am having a particularly difficult session I am rarely aware of the passage of time anyway, but more often than not I eventually just stop chanting and rest for a while in the stillness, and at that point it's like -- I could just sit there indefinitely. There is no reason to keep sitting, let alone a reason to stop.