Well, it's not just hitting a temp mark (210 or 250, or whatever the normal op temp is on your car). It needs to stay there for awhile before the oil can volatilize or burn off the fuel and water entrained in it. At least that's the theory. How long does that take? I would guess at least 20 min, possibly more.
As a practical matter, you can get an idea of what your driving pattern does to the condition of your oil by doing a used oil analysis of a sample taken the next time you have the oil changed. To learn more about how to do UOA's, check out this popular site -
http://www.blackstone-labs.com/. Many people on this forum use Blackstone. In your case, however, I'd recommend the Oil Analyzers lab -
http://www.oaitesting.com/index.htm, because you probably have fuel dilution with your kind of driving. Blackstone does not actually measure fuel dilution, they estimate it using a formula (not particularly accurate). OAI measures fuel dilution much more accurately using an ASTM test method and test equipment.
Once you get the UOA report the next hurdle for a burgeoning oil geek will be interpreting the results. The lab may be helpful in that regard, and you can also post your report on this forum to get feedback from the collective brain (that should be fun).
If you don't want to get into all that, you can hope or trust that your oil condition sensor will alert you when your oil is degraded to the point that you need to change it. You can learn more about these neat devices here -
http://www.machinerylubrication.com/...filter-sensors. BMW uses the Continental Temic oil sensor mentioned in the article.
Or, you can forget all that stuff and just take your BMW out at least once a week for a good, long romp. Get the oil sizzling hot. Kind of the old "Italian tune-up' routine.