Usually after about twenty minutes, the squirrel that is my mind starts careening off the walls in a really unpleasant manner. So I let it out, so to speak. I let my mind wander, drop my concentration on the breath, and just hope for the bell to ring. My ego had convinced me there was an inherent limit to how much sitting the mind could take. It lied.
The body, as a physical thing of muscle and bone, has a limit to how long it may maintain any position with comfort, but the mind is free of such influences. The past conditions of the body, sore muscles, painful joints, carry into the future, but the mind exists single-pointedly in the present. This moment is free. If we wish it to be.
My mind has no inherent limit to the amount of time it can be still. It does not build up discomfort. Thoughts do not actually form like water behind a damn. They come and they go. It is up to me to let them go, rather than trying to hold them back. I gave the squirrel a nut (any decent epiphany tastes good) and told it to sit still. It did.
I sat with greater calm once I realized the limits were of my own making and subject to my own un-making.