There seem to be updates to the ARRL handbook every year, but I don't know if the updates are significant or not. Is the yearly update minor, and buying a new one every 10 years would be enough to avoid missing something important, or is the year-to-year content so different that I should consider buying each year's volume?
I don't think there is a right answer here, but I will share mine and the one I suggest to new hams.
The ARRL Handbook is an awesome reference, especially for the price. You simply get a ton of information from basic electronics on through all the major areas of amateur radio. With the numbering system they use, there isn't even a good way to determine the number of pages in any given edition. I think it's safe to say that it's current page count is north of 1200 pages and has grown steadily over the years.
In fact it was the Handbook that was one of the big draws of amateur radio when I was first introduced to it as a teenager. It was simply the best collection of cool electronic information I had ever seen in one place.
It's too large a product for it to change substantively from year to year; the ARRL just doesn't have the staff for that. But I think it's aim, as a handbook/reference is more like an encyclopedia than a be-all reference. And the basics don't change that much from year to year. What does change are the projects that accompany the articles. These tend to be reprints from QST or other ARRL publications, but none-the-less serve to make concrete examples of the theory presented there.
Technology changes and ham radio follows along (or leads!), so I think one every 5 years is probably fine. I have the last 15 years on my shelf, except for the last two years. That's because I don't buy them new. As much as I like them, $50 a pop is just too big a chunk of my radio budget this year.
But, I probably only spent around US\$100 on the 13 volumes I have by buying them used on eBay and at hamfests. For instance, the last copy of the 2010 Handbook that sold on eBay went for US\$5.00 and the one that's up right now is at US\$0.99 with no bidders.
Shop wisely and enjoy one of the best references out there.