Ldpete, a friend of mine was paying support on three daughters. One aged out so friend went to his attorney to see about having support lowered. The attorney told him that due to my friends current rate of pay compared to his pay at the time support was last set, not only would the payments not be reduced, they would actually increase.
Sometimes it’s better to just be quiet.
Not saying you are in the same situation but I suggest you run numbers through a support calculator before pulling the trigger on this.
And btw: to answer your thread title question:
Never.
While many parents are asked to continue to help (synonym for support) their children financially forEVER, in the bigger sense, as a parent, we are obligated to support our children their entire life. It isn’t always about money though.