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Desktops- about every 5-6 years
Laptops- about every 3-4 years.

And believe it or not, I usually get quite a nice amount for them when I sell them. If one maxes out the upgrades, which I usually do, people will give you a pretty good price for a used computer.
 
apples or oranges

First off i would like to start out by saying I have been a mac user forever. I still have two working apple plus'
My last purchase was an intel mac mini I had it hooked to a 42 inch hdtv. It was the worst thing ever. Evrytime the computer was woke up from sleep the mini didnt output any signal to the plasma. i eneded up using apple remote desktop and chickenofvoc CVOC. i final sold it and now i only have my laptops. My new laptop wont reload mac os. i tried from five different cds. Even the factory install 10.4 not the upgrade. I gave up and loaded ubuntu on it.
Want more info
http://apple.rome-space.com
 
I got my first Mac on July 4, 2005. It was a PowerMac G4 800MHz

Since then, I have had.
Mac Mini G4, 1.25GHz
PowerMac, Dual G4 1.25GHz
Mac Mini, Intel Core Solo 1.5GHz
MacBook CD, White 1.83GHz
MacBook CD, Black 2.0GHz
PowerMac, Dual G5 1.8GHz
Mac Mini, Intel Core Duo 1.83GHz

So, I average a new (to me) Mac about every 3.5 months. Right now, I have no Mac. I'm thinking about getting a refurb iMac Aluminum. However, if I can combine a discount on BF, with my $100 AMEX coupon, perhaps I'll buy a new one.
 
...and I'd buy a new one today if Apple would stop slacking and release an updated Mac Pro. 🙂
 
I bought my first Mac in 1997, my second one in 2002, then one this year. Looks like I'm on a 5-year cycle.
 
Every year for the past 7-8 years. This year is a little longer as I'm holding out for a Mac with a Blu-Ray burner 🙂
 
Each one gets replaced after about five years, but I do laptop, then desktop, then laptop etc. So every two and a half years or so.
 
I'm only on my 2nd Mac, which I got about 6 months after I had gotten a Mac Mini, but I don't plan on getting another Mac until my MacBook fails on me, or I have money to burn. Could be a few months, could be a few years. Certainly within the next 3 years. I don't really want to get another laptop until LED screens are standard in all models, higher color quality panels are used, and there is a significant performance difference. I'm very happy with what I've got right now. 🙂
 
Powerbook 15" g4 TI 667mhz 2001
Powerbook 15" g4 AL 1.67ghz 2005
Macbook Pro 15" SR 2.4ghz 2007

I'm going to try and stick to a 2-3 year cycle from here on
out as I like to always be under the 3 year applecare warranty...
 
I got my very first computer in December 6, 2000. It was a Compaq Presario from RadioShack. From then until February 2003 I got another Presario, an HP Pavillion, and yet another Presario which I used until October 12, 2006 when I bought my first Mac 🙂

I plan on keeping my iMac for many years.
 
Just got my first Mac and first laptop even. 😉 But I followed about a 3 year life cycle for my PC's. I took good care of my computers though, so I never had any real problems with them. So it'll probably be 3 years for my Macs as well as that's usually when your hardware starts to show its age.

Might upgrade every 2 years with laptops since hardware in laptops are already about a year old compared to desktops.
 
I replace my home desktops about every eighteen months. Laptops, about 9-12 months for every category of laptop I have (ultralight, mainline, desktop replacement and more recently UMPC).

The Macs are getting a faster change. My oldest MBP is currently a one-week-pre-Santa-Rosa 17", and I'm changing it to the HD-screened 17" SR machine in the next few weeks. A new 2.6 15" will also soon replace a damaged SR MBP I got rid of recently. Two other MBP's I use for work are early-ish Santa Rosa 15"s. The 2.8 iMac also for work will stay until I replace it either with a Dell Precision running OSX86 or a new Pro, depending on how that machine is.
 
Either when one of my employees loses (or gets one stolen) or there's something I really like that's fancy, new and shiny. Updated chipsets in same old cases don't do it for me.
 
Since 2000:
imac G3 400 mhz
ibook G3 600 mhz
ibook G3 900 mhz
powerbook G4 1 Ghz
powermac G5 SP 1.8 Ghz
ibook G4 1.33 Ghz
macbook core duo 2.0 Ghz

So I guess that's 1 per year (though my macbook is 18 months old now). I had the powermac and ibook G4 at the same time for a few months, but other than that I've never had more than one at once.

I know it seems excessive to replace a computer every year, but the way I see it, it's not really much more expensive. I sell each model while I can still get a good 75% of the original price, whereas if I waited 4 years or something the old one would be practically worthless. Well, that's how I justify it, anyway. 🙂
 
Since 1984: Mac (128K), Mac Plus, Mac Classic II, iMac (Bondi Blue), PowerMac G4 (Gigabit Ethernet), Mac mini. Six machines in 23 years--I wish I had had the money for an SE/30 and/or a Mac II (I was so covetous of these machines!). What a nice trip through memory lane!
 
Since january 2005:

Mac Mini G4 1,25 GHz
iBook G4 12" 1,33 GHz
iMac 20" CD 2 GHz - current
MacBook Black CD 2 GHz - current

I want a MacBook Pro only, but I don't know when I'm going to get it.. Maybe in a few years or so 🙂
 
3-4 years ... and since i added the mac mini in 2005 i'll do alternating every 2 years .. which means next year new PC and perhaps the following year a new mac
 
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