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?**