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My upgrade cycles have been about 5 years, with some overlap.

My first MBP was a mid-2007 model (2.2GHz Core 2 Duo) that I used as my main work laptop until I got a mid-2012 MacBook Air.

This was great as a travel computer (my work at the time involved a lot of airline travel), but lacked power for running VMs or doing heavy-duty development work.

So I upgrade to a re-furbed late-2013 MBP 15 (2.3GHz + GPU) in 2014. I kept the MBA for travel until I gave it to my daughter 4-5 years ago (it's still her daily driver, but getting long in the tooth now).

I still have the late-2013 MBP, but am finding it a bit slow compared to my desktop computer now, and about on a par with my work-provided Dell quad-core i5 laptop, so it sees less use.

The new 16" MBP looks like it ticks the boxes for being a significant upgrade within my 5 year cycle, and I'm very tempted. It should also be an upgrade on my 7-year old desktop Xeon workstation, which would mean it would get more use at home as well.
 
I’m usually on a 6-7 year upgrade cycle. Last one I bought was in October 2013. That worked as well as it did until November 2018 when I tripped over my cord (MagSafe 2 I would add) and it broke the screen. The repair would be like $750 for the screen so I decided to use my TV as a monitor. It has not been ideal. Since January 2019 I have set aside $150-$250 every week to be able to afford the new maxed out 16-inch model.

I’m thinking about putting away $20-30 a week in my savings account for the next 5-7 years so I can get a new one in 5-7 years.
 
Beginning with the PowerBooks in 1999 and continuing with the MacBook Pros, I was on an "every three years" upgrade cycle with my university--until my 2013 MacBook Pro. That machine was so reliable, I kept it for 4.5 years until my retirement in 2018.
 
my last MacBook pro was a 2009 15 inch model that lasted me until this summer, when I bought a used 13 inch 2015 model. I needed a retina screen for a long summer trip, and didn't trust the new keyboards. I'm so glad I got this model, and will keep it till at least the summer of 2021. Looking for a revision 2 of whatever 13 inch (hopefully 14 inch) MacBook pro that comes out with the magic keyboard.
 
3-4 years seems to be my average. I upgraded by 2012 MPB because it actually stopped working. Nothing's wrong with my current MBP, but the keyboard has failed once, so that's reason enough for me to want to get the 16".
 
I have a 2015 13" MacBook Pro.
I am still using that model today because it's super fast and a real workhorse. I purchased a 12" MacBook in 2016 for travel and lighter work, and also still use that. Currently, I have no plans to upgrade to a new Pro unless the 13" goes down.
 
I upgraded my 2012 MBP in 2016 because I wanted to be able to drive a 4K external display. I’m looking to upgrade again this year because 3 years with a keyboard I hate is a long time.
 
I only replace mine when it is (1) too slow for my tasks/work or (2) not working properly with the software I need; or (3) broken.

My first one was 2001 (I think) Titanium PowerBook G4, then 2011 MBP, which had its logic board replaced once by Apple as part of the recall program for dGPU failure, it died again in late 2018.

So my current MBP is 15" that I bought in late 2018 when my 2011 MBP's dGPU failed.

I still have 2011 MBP with dGPU disabled, but it has been delegated to home use only.
 
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