I will be returning from Rome on Oct. 31 and purposely booked a flight that gives us a 6 hour layover in Amsterdam. We will already be checked in on our flight from Amsterdam to Detroit but will obviously need to go through security again upon returning. I've heard that security at Schipol is pretty extensive for trans Atlantic flights. Any suggestions on what time we should be back at the airport?

In a perfect world, an hour would suffice. However, I'd allow 1.5 to 2 hours to be safe. You go through individual screening before you board the plane.

in two hours you will make it, 1,5 you will have to be lucky. Last time I flew to America they interviewed all passengers individually before they were even allowed in the lounge, which was VERY time consuming. Though if you have an American passport, maybe they not so overenthousiastic...

We flew back from Schipol in May 2010, and indeed every passenger in line at the gate (!) was questioned using a pretty extensive list of questions. And it was individually -- first me, than my husband, and we weren't allowed to stand together during it! Easily took 1.5 hours to get through the whole line.

I said "at the gate" -- as Set mentions, you couldn't sit down in the lounge at the gate until you'd finished.

I overslept and arrived just less than an hour before my scheduled departure. I was ready to accept that I missed the boarding deadline. Fortunately I had no luggage to check in and the KLM team was so efficient that they managed to get me to the boarding gate on time to join everyone else for twenty questions.
I would strongly recommend arriving at least 1.5 hours prior to departure even if you already have your boarding pass, but don't panic if you have less, just be prepared to run to your gate!