Three weeks and about 9,000 miles into a five week, 12,000 mile trip. SoCal, north of the Grapevine.
I got into the habit on super long trips of carrying a roll of trash bags with me, and putting them over the dry bags and cinching/bungy-ing them down. It's amazing how dirty, oily, grimy, buggy, bird poopy, and dusty they get on extended trips, esp if your route involves non paved portions of roadway, and it's a cheap, easy way to keep the bags and gear clean and dry. I just change covers as needed along the way and pitch the old ones (in an appropriate receptacle, of course).