Current overwork status: [+] [ ] [ ]
Each box will be filled by one action of overwork, and will take the two turns after that to fade as you recover. The first box incurs no penalty. The second will give a -10 penalty to all actions during the first turn of recovery. The third will give a -20 penalty to all actions on both turns of recovery.
Overwork incurs no penalties on the turns taken, only on the turns recovering from it. You can take as many actions of overwork as you have unfilled boxes.
When you use overwork it fills the left-most empty box, so [-][ ][ ] becomes [-][+][ ], not [+][-][ ]. All boxes recover in parallel and independent from one another, but second and third apply maluses on your actions during that cooldown period.