So, imagine hypothetically you have applied a script effect to some random static or object in a cell. This object was defined in the editor, not created dynamically. Your script set's a variable on said object, and you leave the cell.

How long is that variable maintained? Will it carry over in a save game until the cell resets?

Is it lost the moment you exit the cell?

I am guessing that unless they were specifically created as persistent in the editor they will not maintain their script variables on cell-reload.

The variable is maintained until the object that the script is attached to gets reset. If you never go into that object's cell again, the object will never get reset. If the object is persistent, it doesn't get reset. If the cell isn't in a location that allows resets, then the object doesn't get reset.