I generally purchase a driver ever 3-4 years and woods the same, but between driver purchases.  I just bought new irons, my 3rd set in 20 years.  I buy wedges generally 2-3 years.  And putters have been when I feel the need to make a switch.  Which I've owned about 5 putters, one was for 10 years tho.  I'm always looking, but, I'm short on pulling the trigger until I'm fully confident in the purchase.

Four years for irons, bring on the '16s...

I have found the i get irons every 2 years when the new ones come out because I wear out the center of the face.  Wedges I do at least once a year because they wear out quicker.  For a fairway wood get one you like and stick with it because I have found it hard to get another I like, and driver get one when you feel like you need one.  I also play over 125 rounds a year though.

I play three to four days a week and I purchase new ones when I feel that my clubs are no longer playable.  I just replaced my 910 driver for a 915.  I kept the 910 because it is in good shape.  Wearing out  the center of the irons, I assume you are playing blades.  

Nate - I could probably sell you my clubs as the center of the face is hardly used :)

Titleist wont like this answer, but not very often.  The more you use your equipment, the more you get to know it and how it will work.  I think people change their equipment too often.  However, it's fun to try new stuff, and sometimes the new stuff really is better.

Personally, I skip from club to club pretty frequently until I can get something dialed in. I kept my last set of irons for 5 years and my previous driver for 2. I am playing the same set of wedges for the 3rd season, granted I got them resharpened. Before, I went through 2 sets of irons and 2 drivers a year for a couple of years straight. It's tough to keep the same driver in the bag for a while when they're new techs that increase distance. I also figured which shafts and weights I like and kept those constant. 

I just got my driver replaced to the 915 D2 which was able to add 15 yards of carry. 915F 3 wood, 915 Hd 2 and 3 Hybrids. I like to keep the woods the same company and love the new 915 line across the board.

To me, replacing wedges should be your biggest concern. You hit them the most and they are the most crucial part of the game besides putting. That being said, keeping the groves in good condition is vital so replacing them every year or two is ideal. As for putters, when you find one you like, ride that train as long as you possible can. Irons last four about 4 to 6 years and their isn't much more technology they can put into them to make them better then what you already have. For the metals, replacing your driver every few years is good, especially if you are wearing out the center. Fairways are kinda like a putter, find what works for you and stick with it, probably ever 4 to 6 maybe 8 years if you really like them.

When I can afford it, when I actually need, or a combination of both. Plain and simple.

I don't have a lot of money to outfit myself with brand new gear so I generally buy used.

If I could afford to outfit my bag with brand new stuff, I'd probably do the driver and woods, every 3 - 4 years and about the same with the irons (offsetting the purchases of course. I'd probably do the putter whenever I felt like it (which wouldn't be often) and the wedges every 2 years or so.

Only when they're worn out or I feel there's something better. I just upgraded to a 915 (from a 910) cause it's just a better driver. I still play my 15* 909F2 and my Scotty 2.6. MY irons are CB's and I'll be buying new this year just because of wear. My wedges should easily get me through the year but the 52* and 56* will probably be due next year.