ideally, you have to upgrade your servers once every two years. the idea is to have gained more than what you paid for it and stepping it up just before it starts creating trouble and you end up spending more than what it is bringing in. Also, you need not upgrade your systems every time the market comes up with something new. With two years as your time limit, you still function with equipments that are old but not yet obsolete. Of course, this is an ideal scenario.
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