How many times do I need to put photo block spray on my license plates?

I found a product that works (phantom plate) after a previous one that didn't. That was in May and figured with car washes and rain and just eventually wears out. I've been taking advantage of toll roads to work and back and going to downtown on weekends. I haven't got a citation for using the tolls... show more I found a product that works (phantom plate) after a previous one that didn't. That was in May and figured with car washes and rain and just eventually wears out. I've been taking advantage of toll roads to work and back and going to downtown on weekends.

I haven't got a citation for using the tolls until this month I got one. It said one application and done but nothing lasts forever. So the product Is it because of all the rain in my area? Should I spray it at least once a month then?
8 answers 8