For budget reasons and practicality, for my consulting work (risk assessment, quantitative analysis) I upgrade no sooner than every 2 years. I find that, because I use my Retina MacBook Pro as a workhorse analytically (but hardly at all for email, document creation, messaging--iPad Pro & iPhone 7 Plus for this), an every 2 year cycle, or, even stretching to a 3rd year, assures that no hardware problems will plague me. The older system is handed down to my better half.