My update/upgrade cycle is dictated by the duration of Applecare, which means that I will always contemplate upgrading by the time Applecare (which runs for three years) is due to expire.
In practice, this means thinking about upgrading from about two and a half years; this also means that whatever advances have been made over that period of two and a half years tend to be pretty noticeable.
I bought a MBA in Sept 2010, (a lovely machine, but it was beginning to show its age by early last year) and replaced it with a CTO (maxed out) 13" from 2013. The difference in speed, power, and battery duration are very striking. Meanwhile, I handed over my old MBA (it still works perfectly) to my brother who received a perfectly working computer, with still a few months of Applecare left to run on it, with gratitude.
In practice, this means thinking about upgrading from about two and a half years; this also means that whatever advances have been made over that period of two and a half years tend to be pretty noticeable.
I bought a MBA in Sept 2010, (a lovely machine, but it was beginning to show its age by early last year) and replaced it with a CTO (maxed out) 13" from 2013. The difference in speed, power, and battery duration are very striking. Meanwhile, I handed over my old MBA (it still works perfectly) to my brother who received a perfectly working computer, with still a few months of Applecare left to run on it, with gratitude.