How often you re-proof your FurTech garments depends on usage. For most people once a year will be sufficient, but frequent users may re-proof more often.
I tend to re-proof in the Autumn and before big trips. In the summer it doesn't matter that the outer Durable Water Repellancy (DWR) isn't so good - in fact this can help keep you cool!
Sometimes it isn't necessary to re-proof, just tumble dry, cool iron or hang on a radiator to regenerate the DWR.