Last year, I had a 3 hour layover to Rome. I was able to check out Centraal with no problem. But I almost missed my plane. Luckily, it was on Virgin Express and they let me board 15 minutes before departure. As to Customs, what customs? The customs might as well not have been there in AMS. The train to Centraal took longer than expected. If you haven't been to AMS b4 it's best to familiarize yourself with the train schedule. Personally, it was confusing at first. I had to ask the locals for help. With almost 2x the time, I don't think you'll have any problems checking out Centraal.