It's been 3 weeks since I replaced my thermostat in my 2011 X5 35d. This morning it was -20°c so I thought I'd monitor my temperatures a bit.
My morning commute is 60 km highway driving, it took 20 km for the engine temperature to reach 75°c, 35 to reach 84°c and as soon as I coasted down to 50 kph the temperature dropped to 73°c.
Around town I saw 70°c to 73°c.
This is on a new OEM thermostat, so just keep in mind if you're checking thermostat condition in the winter you're going to need a long highway drive to get a good reading.