I'm fairly new to the game and just curious how often a new mechanic is introduced to the game? On average is it about every other set? (2017 was both GX & Ultra Beasts, 2018 starts off with Prism Star)
I'd say a new mechanic is rolled out about one a year or so. Sets come out in February, May, August and November increments, not counting special sets (e.g. Generations, Shining Legends), and generally one of those will have a fresh new mechanic introduced in it.