Considering the current technological improvement, most of a technological knowledge will be obsolete in a couple of years. One of the best example is SharePoint development. In 2008 mostly ASPX and server side knowledge was required, in 2010 Silverlight, in 2012 Web Service Integration was the focus in 2014 mostly JQuery and Apps and actually who knows what comes in 2016 ? (I guess Angular JS is the new trend but who knows how long),In this sense it makes sense to define the amortization of a technology either in a sense of how much effort does it take to keep the knowledge up to date, or in a sense that how fast will be an existing knowledge obsolete.
Amortization of a software technology:
Definition 1: the average amount of time until an exiting technological knowledge gets obsolete.
Definition 2: the amount of energy that needs to be invested to keep the technological knowledge up-to-date.
In this sense we can speak about technologies with high amortization rate and technologies with low amortization rate. Typically technologies that are pretty knew or at the beginning of the technological curve, like in introduction or early growth phase, have got high amortization rate. On the contrary, technologies at the maturity or late growth phase usually have got low amortization rate.