Flea preventative advice

I am curious as to what others think. I know this app isn't exactly designed for this but how else to ask a group of people besides Facebook which is never very helpful lol. How often should I be giving my dogs a spot on flea preventative? When they go outside they're never in grass. I walk them in areas that do not have grass either. They've never had fleas before but I know they're bad in the area I live in and my next door neighbors pets have fleas currently (it's been hell for her trying to get rid of them)

Usually I give my dogs a flea preventative once a month but someone else told me that's too much. Now I don't know what to do???

What does everyone else think or do? I use pet armor brand if that makes any difference.