About 3 miles and 10 minutes for me in the morning. Our Impala comes up to temp in no time, like, seemingly before it gets out of the neighborhood, but my Mustang takes forever. I know the thermostat is working because you can see the temp dip when it opens and dumps the cold coolant from the radiator into the motor.
Patman what letter on the temperature gauge does it do it on your car? On my mustang it happens as soon as the temp gauge gets on the M and is see it go down to the R. And what temperature is this in fahrenheit since theres no numbers?
You'll have to check it with a scan tool because the gauge is not exactly a calibrated tool. It used to read straight up when warmed up but I broke the sending unit during an intake replacement and the new one reads lower and now it sits one letter to the left of vertical (don't remember what letter that is without looking at it). But, on a scanner it still reads around 190 degrees when fully warmed up. That's where the t-stat opens up too - it'll dip down 5-10 degrees for a minute or two and then come back up again once coolant is flowing everywhere.
The 4.6 definitely takes longer to warm up than many other newish cars I've driven. I believe it is because the 4.6 has a lot more weight to warm up than a typical aluminum V6 or 4 cylinder. You have to remember, the amount of waste heat produced in the engine is roughly proportional to the power generated, so a little 4cyl and your 4.6 will make a similar amount of waste heat leaving the neighborhood at 20mph, but the 4.6 has a lot more "stuff" to warm up.