Would you be starting your trip to FRA from the Köln Hbf or from the Deutz Bhf? If you are starting from the Hbf, how familiar are you with the Köln Deutz Bahnhof? You'd be coming into Deutz from the Hbf on track 1 and leaving from track 11. These tracks are on different levels of the Bahnhof. You'll have 10 minutes to make the change and they say the walk takes 7 minutes (probably less), but you'll need to know exactly where you are going so as not to miss the ICE to the airport. With a Saver Fare, the regional train or S-Bahn to get to Deutz is not specific. That is, you can take any regional train/S-Bahn on that route that day. I might suggest that you take a slightly earlier train just to give yourself time to find you way between the tracks.
Although there is always a chance of delay on any train, I think the chances on a direct train, particularly a high priority ICE, would be pretty good. I worry most about making connections.
In 2001, I came into the airport from Karlsruhe on a direct ICE (about an hour) without problem. Similarly, in 2004 I came in on a direct RE from Boppard (a little over an hour).
If you really want to be sure, come in the night before and stay near the airport. Downtown Frankfurt and Mainz are at either end of S-Bahn lines that go through the airport Regional Bf. I've stayed in Mainz three time before a morning departure. I like Mainz because it almost as convenient as from the Frankfurt Hbf, but I think it is a little less pricey. I stayed here the last time. It's a short walk across the Bahnhofsplatz to the S-Bahn to the airport.