Calcium oxide is being proposed as a regenerable sorbent for CO2 at high temperatures via a carbonation/calcination loop. It is well-known that natural sorbents lose their capacity to capture CO2 as the number of the carbonation/calcination cycles increases. The equations proposed in the literature to describe this decay are limited to moderate calcination temperatures (typically well below 950 degrees C). This work studies the effect of the calcination temperature and calcination time when the temperatures exceed 950 degrees C. Such high regeneration temperatures are necessary in some of the most efficient processes proposed, and they are shown to strongly affect sorbent performance. We propose a model to describe maximum sorbent conversion as a function of the number of cycles, the calcination temperature, and the calcination time. This model combines existing knowledge about the decay of sorbent activity under moderate calcination temperatures with the effect of sintering in each cycle.