It depends. Replace your plates only when they start to get thin (or unless you have UHF Originals, in which replace them immediately with something decent). Brand new plates will slow down after a decent session on handrails, and after a few grinds on some curbs. They'll also slow down as they get clogged with dirt and old wax, so make sure you scrape it off each time you get a buildup of skank

Wax both prolongs the life of the plates and gives a smoother ride. Unless a curb or ledge is naturally slick you shouldn't need any wax; granite, for example, slides really well on it's own. Generally avoid trying to grind really rough stonework, since the amount of wax needed to smooth off the surface is ridiculous. Painted surfaces generally don't need wax