It varies wildly from game to game... I go as far or do as little as required for a particular game or circumstance.
I generally view the game rules and all the various things in the game as a starting point from which to add, trim or change if and as necessary.
I generally use the rules as written - I'm more likely to ignore some of the minor things (arrows, rations, juggling action economy) than to add new rules or rewrite whole sections of them.
I rarely ever ban spells, items or classes from official products or UA articles.
I don't generally use third-party classes unless it's absolutely impossible to model a character concept with the existing ones, but have no problem tweaking the existing ones (sometimes severely) to better fit individual tastes. (And I will unrepentantly drag the multiclassing rules into a dark alley and do unnatural things to them in order to make a character concept work the way a player wants it to...)
As far as campaign worlds, or new items, spells, monsters, etc., I freely steal or borrow from anywhere and everywhere and am constantly coming up with new stuff that I may or may not ever use. As far as I'm concerned, "Canon" is a misspelling of the big metal thing I use to blow established settings and other game elements into little pieces I can build other things out of.
For the most part, though, across the various editions, I've generally found that I didn't need to change very much that was already in the game unless I was intending to do something wildly different on a grand scale.