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I've had 2 MBs, a 2006 CD MB and 2010 MBP, both 13".

I sold the CD MB in 2012 to pick up the MBP used, mainly because I was moving to iPhone and OS X at the time was limited to I think Lion. For most things, it took care of what I needed, but it struggled browsing the web.

I would still be using the MBP but my mom's PC died so it went to her. I picked up a mini to hold me over until Apple launched an awesome MB, or MBP. I am still waiting.
 
I have had two Apple laptops. The first was a MacBook, the 4,1 model which I purchased the week they came out. The second is my main computer still, the high-resolution matte display 9,1. It's been upgraded to have 16GB or RAM and a 2 TB HDD. I use it almost strictly as a desktop, almost permanently plugged into daisy-chained hard drives, my speakers, and my display. I don't believe I'll get another laptop. Whenever I get another Mac, hopefully a long time from not, it'll be a maxed out iMac.
 
It has grown over time. Use to be 2 years, then 3 years, now it'll be 4.5 years. Processors and junk are so fast nowadays, SSD is a godsend, and I never need more than 16GB of RAM. Unless some killer tech comes out like quantum computing in the next decade, my new rMBP this year (please...) will probably last me at least 5 years.
 
I tend to get a 'new' primary laptop every 3 years or so. Generally, my laptops are purchased used; this allows me to keep up with advances while keeping my costs down. I purchased a 12" PowerBook (1 year old at purchase) when I started college, graduated up to a 2009 Air (2.5 years old at purchase) after I graduated, then to a 2012 Air (2 years old at purchase) after a few years, and now I have a 2011 11" Air (3 years old at purchase) as my main laptop.

The PowerBook was my longest-lasting laptop at just shy of five years. I actually still have it, and still use it occasionally in my classroom.
 
A minimum of 4 years. Got my first one in 2007. Upgraded in 2011. Upgraded again in 2014, earlier than I wanted but needed more than the 128gb that mbp had. I've had my rmbp for 2.5 years now and it still runs like brand new so I'd like to think it hasn't reached half it's lifespan yet. Honestly I don't use it all that much. My iPad Air 2 is my main device. It's my 2nd one, the iPad 3 being my first. I've had it a year now, hoping to get at least 3 more out of it.
 
Every 6 to 8 years I get a new desktop solution.
And then the machine still works, but it gets demoted from creative box to writing on the balcony machine.
 
I have a 2015 rmbp, will probably upgrade if this refresh has 64gb ram. 16gb ram is piddly nothing when browser tabs take up 100mb each and apps can take 2+ gb. I should probably get a pc because I need the horsepower but am waiting for the macbook update to decide.
 
So far, about four years. I've got a 13" Mid-2012 now. Might be able to stretch this one out even longer; kinda depends on whether it'll get left behind after Sierra stops getting updates.

Swapping the optical drive for an SSD would give it a kick in the pants, too.
 
I had a 2009 MBP to start me in high school, and a 2013 rMBP to start me in college. I will be upgrading to a 2016 rMBP, so 3-4 years is my pattern so far and I would suspect it'll stay that way.
 
Currently have a mid-2014 13" rMBP bought brand new that's running all my stuff. Prior to that I had an original 2008 Rev A 13" MacBook Air.

Usually can't justify upgrading until they are borderline unserviceable or become unsupported by the latest software.
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Still using my Late 2011 MacBook Pro 17"
Always wish I had the finances to grab one of these at the time.
 
I'm not upgrading much these days. Every 3-5 years or so. My pc's are older than my Macs and I use them more. Decided against a new Mac and for my usage bought an iPad Pro.
 
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