I've looked around and haven't seen anyone else complain about this so I am wondering if it's just me. I have a 2013 A4 that I've had since new and it's always taken a long time to warm up when it's cold outside. Yeah, I know it's going to take longer when it's cold out but I also drive a VW Tiguan which has an obviously different but still similar 2.0T engine that warms up in less than half the time. I recently brought it to the dealer to have it checked (under warranty) and they said there is nothing wrong. Interestingly, the 2.0T in the 2016 A3 they gave me as a loaner also warmed up in less than half the time. I took a video of it the other day on my ride into work. Total time to reach operating temperature was just over 18 minutes and the temp gauge can be seen going up and down during the warm up cycle. The outside temp was hovering right around 0 but the car came out of the garage which was about 30 degrees. 18 minutes doesn't seem right to me. Any thoughts?

 

At first I was going to say it appears normal as my S4 (3.0t) takes about 15-20 minutes to warm up as well (less time if I shot off the heater) in freezing weather. However I saw that you mentioned it goes up and down during the warm up phase? That should not be happening!
Usually that indicates one of 2 things:
1) Most likely your Thermostat is either stuck open or won't fully close. This will cause the engine to take longer to heat up and show fluctuation. Not only is it annoying it will affect gas mileage/emission
2) There is a chance your temp sensor is not OK anymore and causes incorrect readings

I'm surprised that this would happen on a 2013 but you can always have a weak thermostat/sensor even in a brand new car.