I can now verify that the "Pitot Overheat" warning appears at exactly two minutes after activation. The problem with that is I was just preparing to taxi onto the active when this happened. I move thru the checklist fairly quickly, but that pushes my average on the last before-takeoff checklist and I would rarely make it. Also, that is sometimes impacted by clearance delivery delays.
With that in mind, I am amending the checklist (for my purposes) to include the pitot heaters just prior to the takeoff roll.
As for their actual performance, I did experience icing today descending over Germany, but the period of icing was fairly brief, I turned off the heaters but the airspeed indicator didn't fluctuate after reactivation. There may not have been enough time to create an impact. When I get a period of continuous icing in a future flight, I'll test them again and will pass on the results.