I purchase a full set of dice once or twice a year. I also buy loose dice whenever I have some pocket change and am actually in a store that sells dice, which is not all that often. I recently acquired three D6's that have roman numerals instead of numbers or pips to complememnt the one that I bought in California two and a half years ago.
I chose "every year," but in truth it is a bit more complicated than that. It has been a year or two (or perhaps even three) since I last purchased any, but there was a span of a few months about five-six years ago when I purchased a set every few weeks. It was a phase I was going through, I think. So on average it is probably about once a year, as it has been a couple years or so since I last purchased any.
One of my players from the high school game I DMed gave everyone a set of dice for christmas, and I still use those. They are awesome. But that was over ten years ago.
I purchased a set of dice to match them (ivory to match the onyx set I was given) about six years ago.
I've been thinking this past month that I really need to just go out and pick a new set of dice to use, lest I wear out my current beloved set. I just can't stand the feel of new dice compared to my worn in set, but of course i don't use the new ones enough to wear them in sufficiently....