My expectation is a minimum of 5 years, up to 8-10 years.
That doesn't mean you won't consider an upgrade before then, but that any system you're paying over $1K for should last at least that long physically...or it's time to find a new manufacturer. There are always exceptions (going through one right now w/wife's 2012 MBP, but otherwise all of my Apple gear has made it 5+ years), but it shouldn't be a 'normal' expectation that a laptop only lasts 3 years before needing replacement, especially those of us buying maxed out $3k+ systems.
My white Sandy Bridge macbook is still running plugged into the TV. My old iPhone 3GS I used to use solely for music for off road motorcycling. My early 2011 15" dGPU MBP is still chugging away (added SSD, second drive, + 16GB) as my desktop replacement for software work, connected to 1 or 2 external displays depending on home or office, with it's only real 'failing' being crap battery life.
I typically run a 4 year 'consider upgrade' cycle for laptops, and 2-3 years for phones. Sometimes it's been slightly shorter when there was a real technology jump I just couldn't do without (e.g. Core -> Core 2 Duo, then to quad core), sometimes longer (waiting on next release to consider replacing my 2011 MBP, but haven't lost very much in way of performance, just not retina, which external displays + matte 1680x1050 laptop display makes up for well enough, for me).