T.S.
We don't recalculate at any kind of time interval, only if there's been a "material change." It isn't worth the filing fees and attorney charges to have the support raised or lowered by a few dollars a month.
I asked for support to be recalculated several times in the first couple of years of our separation-->divorce because we were in court anyway, so like pp said, might as well. Parenting time and custody was being reworked constantly so it always made sense to rerun the calculator with the new amounts.
I ended up having full physical custody of my son and ultimately moved out of state with him, so we calculated it one last time to reflect that and it's stayed that way for the past few years. His father certainly makes more money now than he did then, BUT he's gotten good and paying the set amount regularly, and I'm not strapped for cash, so the amount he pays seems like a reasonable contribution to the support of his son (the bulk of it ends up in a college savings each month, actually) so I don't see the point in filing a new motion... plus, we don't even LIVE in the state that does the support anyway, so it'd be a hassle...
No, I don't ask for his tax returns. I don't ask to see his income. (although he's a public employee so if I was interested, I could look it up on his salary schedule).