How old is your main Mac? Since I started using Macs 20 years ago, my provisional target interval for system upgrades has been three years, and in quite a few instances I didn’t stretch it out even that long.

However, my current number one workhorse Mac just passed its third anniversary last month, and it actually has had a longer life than that, since I purchased it as an Apple Certified Refurbished unit, although there was no indication upon unboxing that it had seen previous use.

Anyway, this late 2008 model aluminum unibody MacBook (same form factor as the current 13-inch MacBook Pro) has been a rock of dependability for three years, literally never missing a beat or manifesting so much as a reliability hiccup in what has been pretty intense use on a daily basis, and it still looks like a new machine.

Consequently, I don’t find myself having much incentive to upgrade as we head into year four. The MacBook has a Core 2 Duo 2.0 GHz CPU and a  NVIDIA GeForce 9400M integrated graphics processor, so it will support OS X 10.7 Lion, and probably the final version of OS X 10.8 Mountain Lion as well (although I’m still running OS X 10.6 Snow Leopard, which is stable, fast, and can still handle both Carbon apps with Power PC code and supports my Apple USB modem for use during power and wireless broadband outages—both of which are, unfortunately, common here in the outer boonies). Lion will do neither, which has so far been a deal-breaker for me, although I expect that if I do end up running the MacBook for another year or so before upgrading, I will probably try installing Lion on my second hard drive partition for compatibility reasons when needed. However, I currently have no intention of changing up from Snow Leopard as my workhorse OS.

The only other upgrade I’m contemplating as part of a life extension strategy for the MacBook is possibly upgrading the RAM to 8GB. I currently have 4GB installed, but I hear form other older Core 2 Duo Mac owners that going to 8GB makes a noticeable difference in liveliness. Speaking of which, while my MacBook is far from the bleeding edge, it still feels reasonably quick to me. A larger capacity hard disk would be nice as well, but is not crucially necessary, since I still have some headroom (about 25 GB) left on the main partition of the current 160 GB unit.

Another factor weighing on my own deliberations on the matter is having bought an iPad 2 nine months ago, which I’m using as a work platform more than I thought I would, and I expect that might also be the dynamic for others with older but still serviceable Desktop and laptop Macs.

I’m wondering whether there are many other owners of middle-aged Macs who are finding themselves as satisfied with their three or four year old machines as I am, and delaying system upgrades accordingly. I’m thinking there probably are.

The Houston Chronicle’s Dwight Silverman thinks so, saying in a recent column that had someone told him five years ago they owned a 5-year-old computer, his boilerplate advice would’ve been to replace it with a newer one. However, he allows that today the situation is very different, with 2007 vintage system hardware remaining fairly robust, since Intel was already marketing Core 2 Duo CPU silicon back in ’07, and dual-channel memory was prevalent. Consequently, 2007 model and later computers can still typically handle most of the application software and operating systems available today, a point to which Silverman can empirically attest, since his main personal computer is a a mid-2007 model iMac modestly hotrodded with a 6 GB RAM upgrade, which is not officially supported, but which he says works fine.

Silverman says he didn’t originally expect to hold onto this iMac for five years, and historically, like me, he’s been inclined to buy a new primary desktop computer every three years, but this time he’s not yet felt the need for a replacement.

There’s more than a bit of irony when we Mac veterans wax nostalgic for how great we imagine the old days Macs were, and that’s a largely accurate recollection, albeit with some notable exceptions. However, my last two Mac laptops—a 17-inch G4 PowerBook and the MacBook—have been without question the two most reliable and dependable computers I’ve yet owned, and it’s arguable that the current aluminum unibody MacBook Pros and MacBook Airs are the best laptops Apple has yet come up with…or at least they would be if they had user-replaceable batteries.

How about you? What’s your targeted system upgrade interval, and have you been stretching it out recently?