At some point you start running out of things to buy. At least, things you really need. A test to see if you're close to that threshold is to ask yourself how many of your tools you truly understand in great depth, versus how many you haven't gotten around to fully exploring.
I'll admit to having heard a demo of a great sample library and thinking I might want to acquire it, only to find out that it was already sitting on my hard drive. Or realizing that I could easily duplicate the characteristics of some effect with equivalents I already owned.
All I have to do is watch one of Dan Worral's tutorials on any tool, even those I've used for years, to remind myself that I still don't know everything about all of them.