Ideally, we recommend using Universal Protector once a week.
All shoes are different, and all will suffer through a variety or conditions. By proofing at least once a week you can be sure to keep your footwear looking at its best for as long as possible.
If you're caught in particularly heavy rain, or if you've just polished or cleaned your shoes, however, it might be worthwhile to spray your shoes more regularly.
The best way to ensure that your footwear is able to protect you is by pouring a small amount of water over them. If the water beads up and rolls away, your shoes do not need proofing. If any water is absorbed you should reproof your footwear to ensure that they're able to protect you.