I have never roasted decaf in my Gene, but have "heard" that they are harder to roast from hearing the crack and monitoring the colour.
Try a well processed, *large bean, non decaf bean to start with.
Depending on temp / profile / your Gene, you will hear FC as a fairly pronounced popping. You wont mistake that. *Assuming your FC stops and you get to SC, it will sounds more like the crinkling of paper, softer cracks.
Again - depending on all the variables, *on average I get to FC around 11-12 mins and SC around 14-16 mins