I'm interested in this question, too. On average I brew about 12-16 times a year, and when I bought my first bottle of campden tabs in 2014, it was a bottle of 110........so. Using a quarter tab in both mash and sparge water, I'll still have campden tabs in say.....2027 to 2032!
Also when I use a liter or more of tap water for my starters I'll very unscientifically "shave" a bit off of a Campden tab into the water prior to heating. I have no idea if this is helpful, or even detrimental.