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Let's see:
1998: iMac G3
2002: iMac G4
2006: Macbook Pro
2011: iMac
2020: Waiting for this year's iMac 😊

So 4-4-5-9 is roughly every 5,5 years.

Funny I updated more often before I had a fulltime job (<2007)
 
We upgraded a late 2009 iMac to a 2019 model last October. I had added memory and SSD to the 2009, and it was still reasonably functional, but was showing its age with slowdowns on many web pages and that sort of thing. We could have kept the 2009, but the money became available and the desire was there. (Also, the CPU power jump from 2009 to 2019 is much larger than the relatively small jump from 2013 to 2009. Intel has been stuck in a rut essentially since Sandy Bridge.) The display, GPU, and storage speeds are much improved since 2013, the CPU not nearly as much.

Basically you upgrade when you need to, or want to and can afford it.
 
Haven't heard from the OP in a while, but the results from everyone's input isn't surprising.

There is a range that people upgrade their Macs range from 18 months to 14 years, all depending on the individual's requirements and circumstance.


I own a late 2013 27” iMac and although its running without issue
To the OP: If you are happy with your Mac with no complaints, I wouldn't bother upgrading. You said you do basic things with your iMac, which "basic" needs to be defined, but if it is just simple web browsing, your iMac should be able to do that for many more years.

Not reason to drop $2k on a new iMac if your current one works great.
 
Let's see:
1998: iMac G3
2002: iMac G4
2006: Macbook Pro
2011: iMac
2020: Waiting for this year's iMac 😊

So 4-4-5-9 is roughly every 5,5 years.

Funny I updated more often before I had a fulltime job (<2007)
My experience was similar, except for the "full time job" part.

I think a lot of people are keeping Macs longer than they used to. I think one of reasons is because the model-year differences of the intel Macs isn't as significant as the PowerPC Macs, especially with Intel's Core "i" cpu.

I own a lot of Macs, but for my main ones, meaning the primary one used at the time, I had this upgrade path:

1995: Performa 631CD
1999: Power Mac 6300
2001: iBook G3, 12"
2004: iMac G4, 17"
2007: Mac Pro 1,1
2012: iMac 27" i7, Max-out BTO with the exception of RAM which I did myself, and a 1TB Fusion Drive

I remember going from non-PPC Performa to a my first PPC, then my iBook G3, the performance difference was very significant.

The same when going from a iBook G3 to a iMac G4, even though they were just a two years apart in release date, the performance differences was very significant.

The MP 1,1 was also a huge performance difference, but after that, not as much.

While the 2012 iMac was a nice upgrade over the 2006 MP, it wasn't as significant when compared to past upgrades.

I originally planned on upgrading my 2012 iMac around 2015-2017, but the performance differences overall isn't worth it for what I do with my Macs. I might get a 2019 iMac, depending on what the next one is like. If the next iMac has soldered everything and a T2 chip, then I will try to find a decent 2019 iMac.
 
2007 - iMac 17”
2011 - iMac 21.5”
2015 - iMac 27”, sold 2019

Seems like I go exactly 4 years with every desktop. I think that’s quite reasonable for a light user.

However, I have decided to not even consider another iMac until they get a redesign. I find it feels outdated. It should offer some sort of biometric security. It should ship with SSD:s as standard, all laptops do, and even the base Mac Mini. There is no reason why it shouldn’t be at least a no cost option on the iMac. Then there’s the bezels...
 
...
However, I have decided to not even consider another iMac until they get a redesign. I find it feels outdated. It should offer some sort of biometric security. It should ship with SSD:s as standard, all laptops do, and even the base Mac Mini. There is no reason why it shouldn’t be at least a no cost option on the iMac. Then there’s the bezels...

I'm all in favor of going all-SSD, but unfortunately there very much is a reason to ship base models with hard drives: hard drives are still much cheaper, and they're going to stay that way at least for another couple years. Flash memory oversupply was depressing SSD prices, and even before the COVID-19 effects, memory manufacturers were cutting back supply to stabilize prices.
 
I used to sell every two years, but my 2017 5K with 32GB/1TB still does absolutely everything I ask of it so I will probably hold onto it until something fails (it has Apple Care through this year).
 
I'm all in favor of going all-SSD, but unfortunately there very much is a reason to ship base models with hard drives: hard drives are still much cheaper, and they're going to stay that way at least for another couple years. Flash memory oversupply was depressing SSD prices, and even before the COVID-19 effects, memory manufacturers were cutting back supply to stabilize prices.

Sure, but the iMac is not a cheap device. No reason why the 256 GB SSD shouldn’t be a no-cost option.

Heck, even the new gaming consoles will be getting 1TB SSD:s. I doubt they will even cost a third of a 27” iMac!
 
I used to buy iMac and MacBook laptops every 2-3 years. 2007, 2009, end of 2012 (iMac 27 inch major redesign) and 2013 MacBook Air with 256gb/8gb ram

i went 5.5 plus years before replacing mba i got for $1500 with surface pro 6. I sold surface pro 6 2 months ago after owning it for 12 months. Just purchased 2020
MacBook Air

but my late 2012 27 inch iMac With 24gb ram with 128gb fusion ssd drive still performs well. I mainly use it for spread sheets. Basic stuff. It’s works fine. I don’t think i will ever spend $2600 on another desktop computer again.

I don’t want to spend more than $1000 on laptops either these days I used to spend $1500-2400 on MacBook Pro laptops as well.
 
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