How often do you check your Truck's tire pressure?

Well I have noticed that there is a pressure loss in my 19.5s that is faster than with my 16s. Enough of a loss that I bought a quality digital pocket sized tire gauge just to verify that it is a real loss vs gauge accuracy issues. I had them at 100/85 when I left for my trip. 2000 miles (20 days later) it was 96,94.5/84.5,81.5. I have since reset them to 100/81.5 and will check weekly and before any trip. Wanted less in the fronts so I just matched to the lower of the two.