Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
5 years.
I have a 2011 13" Pro, upgraded to 8GB of RAM, a 128 GB SSD and the original 320GB HDD in the optical bay. I am confident that it will last through the end of my student career (3 years) and I've had it for 18 months now.

But I think I will change when I'll have enough spare cash and the market will offer me hi-res displays, 512GB-1TB SSDs and CPUs-GPUs powerful enough to feel a more than noticeable difference, on a budget of 1000€, more or less. That includes PCs as well.
 
I finally caved in and replaced the 2008 Macbook I referred to earlier in this thread. I now own a late 2011 MBP. The real reason was I wanted to run Mountain Lion on my "main" machine rather than only being able to run it on the mini I share with Mrs. r0k.

How long have my Macs lasted? I still have my 2005-ish Mac mini. I don't power it up these days but when I do power it up, it works fine. As others in this thread have already said, a Mac lasts as long as it does what you need it to do. Sure there are isolated problems like quality issues on some Macbook Pros a few years back but these are the exception rather than the rule.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.