An MD80 First Officer reported forgetting to retract slats during departure and exceeding the slat limit speed. A long duty day and fatigue are cited as factors.

Narrative

I signed in very early for a five-leg day. Aside from a MEL that required maintenance to run the flaps full down then up verifying every position and then back down to takeoff position prior to every gate departure; with a visual confirmation from me down in the wheel well; it was just another day. The weather for most of the arrivals and departures was lousy; low overcast; rain; thunderstorms; deviating and a bit of holding. Combine the weather and the MEL requirement and our twelve hour duty day was running long. By the time we departed on the last leg; we were over two hours late. I was the pilot flying; the takeoff and departure was normal. Out of 10;000 FT I set up for a climb at 280 KTS for the soon to be bumpy ride. Somewhere between 12;000 FT and 14;000 FT; the over speed cracker sounded. I looked down and saw that I never called for the slats to be retracted. Immediately; I called for the slats to be retracted. I would say that the clacker sounded for just a few seconds; and I believe that I never exceeded 280 KTS by more than a knot or two and that was the IAS I was flying. After landing; the log was filled out with the entry of the slats over speed. I am still beating myself up over this; and this is not how I fly. The incident happened at about just over thirteen hours of being on duty. Our total day ended with fourteen hours and a minute on duty. Sometimes it is hard to determine when you are fatigued; when you ARE fatigued. Looking back at it; it is obvious. Looking forward now; I think I will just have to stop what I am doing around the twelve to thirteen hour mark and ask myself how do I REALLY feel.