In "Undoctored", Dr. Davis recommends 6 weeks of probiotics to reseed your bowel flora then stop. If you are eating clean and including prebiotic fibres in your diet you shouldn't have to take them again unless something comes along and disrupts things, like a round of antibiotics.
I have taken a 60 day bottle full on a few occasions - once on the start of my low carb health journey, again when I decided to eliminate artificial sweeteners (they supposedly disrupt your gut bicrobiome) and again right now as I am switching to all dechlorinated drinking water. I figure if I am going to the trouble of doing things to heal my gut, I may as well give it the best chance. I don't think it can hurt anything but my pocketbook, but a bottle of NOW 10 billion, 9 strains is pretty cheap insurance to my mind.

Have you read Undoctored? Davis gives an easy, well explained protocol for general good health, including parctical advice on supplements (he is pretty restrained on this subject). Recommend highly.