The basic canvas bags that I've seen (back to the Baggins and Sackville bags) all have enough of a base treatment to keep things waterproof. I had gone to the double-seal system when I got my Country Bag as my older Banana Bag (which saw a lot of trail/mixed-terrain miles with no rear fender in place) had gotten really, really grungy. When I started cleaning it off, it seemed like the mud and dirt had gotten into the fabric. From my experience in owning Filson/Barbour garments, mud and dirt clean off better from a slightly thicker coating. I had gotten a bit of seepage of rain at the seams, so wanted to make sure that since I was using the bag for longer rides and possibly randos that it was well sealed.
Retreating is really necessary only when you start having seepage - usually evident at the seams first, depending on the bag's construction. With my jacket that sees the most usage, that's maybe every 2 - 3 years, but there are a lot more moving parts and seams exposed directly to the rain than with most seat/saddlebags.
- Jim
cyclofiend.com