I bought my MacBook Pro in mid-2009, and wanted "at least six years use out of it". Mainly because I was only 16 at the time (I'd saved up from my washing up job in a caffe for years 😀) and funds were very low (it cleared out my bank account).
After almost five years of very heavy use (it travelled to/from college with my nearly everyday for two years, and has done the same for me at university for the past three) it's still going strong. The only thing that's wrong with it now is the battery, which only lasts ~45 minutes on a full charge now (it's had 1205 cycles).
I plan on keeping it until at least this time next year, if not longer. If it does give up on me before then, I'd still be really happy with how well it's served me.
But I seriously can't wait to get a rMBP. Those things are so fast compared to my mid-2009 machine it's laughable. Good times to come!
EDIT: People also laughed at me when I said buying a Mac is value for money. My flatmate has spend £1300 on Windows PCs in the past 4 years... for a hundred quid extra he could have had a Mac, saving money and having the pleasure of using one!