Is creamed butter stable for mid or long term storage? I find creaming butter a pretty annoying process.
I would like to make lots of it at once and aliquot for later use.
Is creamed butter less stable than butter? You wouldn't think so because you are mixing 2 ingredients which virtually have no water (butter and sugar). Does it separate over time?
If indeed it is stable, is there a reason why grocery stores do not sell it?
If you are asking about butter creamed with sugar, you can do this.  I would place in an airtight container, maybe with a piece of plastic or wax paper on top, and refrigerate.  It should be good for a couple of weeks.  Of course, you will need to bring to room temperature to soften before use.