I'm not sure anyone answered the OP's precise question, which was how many hours in advance of the flight departure can you check bags at KLM in AMS?

I flew AMS-BOS-CLE in May and had a checked bag. My flight departed around 1445; I arrived at AMS around 1100, went to the Sky Priority check-in area and checked my one suitcase. The set up at AMS is that you wait in a line to be admitted to the check-in area, two KLM employees were guarding the entrance to the check-in area; I showed them my passport and my BP on my Iphone and they admitted me to the area. After dropping off the bag and getting a claim check, I then proceeded to the security entrance (to get the expedited security at AMS you have to go through the entrance in the Sky Priority area. You then have to place your BP on top of the gate to get to security.

So the real question, in addition to how early can you show up, is whether once you check your bag you can leave the area and go into Amsterdam, or whether you are compelled to go through security. Sitting here, I don't know the answer.