My seats usually take about 2 minutes to warm up and 3 or so to get scalding hot with an ambient outside temperature of around 40°F. They still get much hotter much quicker than any car I've owned with heated seats previously (Acura, Nissan)
Keep in mind the seat time to warm up is most likely going to depend on a whole lot of factors...how cold was the foam in the seat to begin with, how much juice is coming out of the alternator, how fat your rear end is, etc.
Current: '15 M235i Vert
Previous: '13 335i, '08 TL-S, '00 Corvette