Using a Graham scanner, I've been looking at the O(2) trim, which appears to be a good indication of when the vehicle is throughly warmed up. Even using a block heater, the warm-up interval to a quasi-stable, zero-centered mixture appears to be about 30-35 minutes. Although the block heater lets the ICE reach S4 a little quicker, about 5 minutes, zero-centered mixture trim takes longer.
Bob Wilson