From my experience with WP/B garments, keeping them clean will keep them useful and functional for a long time (I have a 11 years old Gore jacket). What I do is:
- Clean the garment in the washing machine (30 degrees, delicate cycle) using pure soap flakes under two conditions:
- Heavy soil (WP trousers after a very muddy trip) will be cleaned straight away
- Regular use - wash after 10 days of use in total (can be multiple trips)
- Apply a spray on DWR (Nikwax/Storm/Granger) after every wash
- Wipe excess with a cotton cloth (also spreads the spray on)
- Put in the drier for 20 minutes on very low heat
So far that worked very well for me in maintaing all my shells. I use the same principles for wind jackets (for cycling/running) and softshells.
Note about softshells (weaves): Make sure you give them a second rinse as the soap tends to get really stuck in the fabric and than continue with the spray on and drier.
In the first 60 days of actual use (first year for heavy use, 2-3 years for less frequent use) I won't apply the spray on, as the factory DWR will hold with just a wash and heating.
If you don't have a drier, just place the garment on a warm radiator for 30-60 minutes (depends on heat) or use VERY CAREFULLY a low heat iron. If you are not sure about the iron don't use it!
I just wrote an overview the other day about waterproofs if you need some more in depth information.