Before I get into the topic, there is one thing to keep in mind about catalogs. Catalogs take a long time to publish. There are thousands of catalog listings spanning hundreds of pages. By the time a catalog is printed and shipped, it's already a few months behind the constantly changing market prices. There is no printed catalog that can possibly keep up with the latest market conditions. That is true of all catalogs, not just Scott.
With US stamps, the Scott US Specialized catalog is the bible. My comments will be in regards to the US Specialized catalog. I'll probably alienate the catalog publishers. But here is my opinion on buying catalogs.
If you're investing in stamps, buy a catalog every year. In fact, you need more than just a catalog. Using Scott Stamp Monthly, the PSE Quarterly Market report, or other sources will keep you more up to date with current values. You can also track the prices realized at major stamp auctioneers to see what the current market is for certain material. If you're investing in stamps, your need for information probably goes beyond just the Scott catalog. The latest catalog is a good base point to start at though.
If you're not interested in tracking the absolute latest catalog values of the more valuable US stamps, you can probably get away with buying a catalog every few years. For new issues from the USPS, Linn's Stamp News and other philatelic publications periodically list the new catalog numbers as they are assigned by Scott. Keep up with the newest catalog numbers that way. When I was collecting stamps (and not dealing), I bought a new catalog every 3 years. That kept me current enough for my own needs. Prices on cheap to medium range material don't fluctuate that much to warrant buying a new catalog every year.
If you're not interested in the more valuable US issues and you're not collecting the new US issues, you may not need a new catalog at all. In general, the catalog values of material from 1940 to date haven't changed much over the last several years. These are largely inexpensive stamps and they will be inexpensive for generations to come. Even the few better ones, like Scott #1053 or Scott #834, haven't changed much in the last several years. If you're not worried about expensive material and you can ignore the new issues from the USPS (or you're getting the catalog numbers from some other philatelic publication), I don't think it's worth your money buying a new catalog only to find that the stamps of interest to you have only changed in value a few pennies here or there.
There are some people out there who insist that only "serious" collectors buy a new US Specialized catalog every year. Hogwash! Yes, there are some collectors who should buy a catalog every year. But there are other collectors out there who don't really benefit by buying a new catalog every year. Sometimes, an older copy suffices quite nicely.
A new catalog will lighten your checkbook by about $60. For collectors on a limited budget, an affordable alternative is to buy last years catalog when the new one comes out. Dealers look to unload their last few copies of the previous years catalog at bargain prices when the new edition comes out. Or you can find lightly used copies at large discounts off of the full retail price.
In summary, buy a catalog as often as you feel necessary. You can buy the latest catalog. Or you may consider buying last years catalog at a reduced price. Let your needs determine when it's time for a new catalog. Having the latest copy of Scott on your desk doesn't make you any more or any less of a philatelist.