Trying to Meditate Longer I am currently meditating thirty minutes per day. I wonder whether I should meditate an hour instead or meditate for thirty minutes twice. 

Basically, I meditate on compassion and after thirty minutes I'm not generating much of anything. I just sit there moving about on my chair, wondering about the time, etc. 

Does this saturation after thirty minutes occur because I have always meditated this amount of time before? Or is it because I'm just not ready yet? In other words, are we used to whatever amount of meditation we're usually doing?

If this is so, would it be better to just always try to sit an hour if I'm capable? Should I just meditate as long as I can sit, whatever the quality may be?
Joggers or long distance runners didn't just start out running for hours. They build up their stamina slowly and steadily over time. So use the same approach for meditation. Build it up gradually and don't settle for the 30-min achievement. It's probably too short for any significant deep absorption state anyway. There're advanced meditators who immerse themselves in deep absorption for days.