However, ideally, longer than 35 months would allow for all of the retrofits to occur over a period of several years so that facility owners can properly plan outages and suppliers can properly plan for resource availability.
Retrofits have not traditionally taken longer than 40 months. 
mnli hypothesis: Retrofits have not traditionally taken longer than 40 months.  premise: However, ideally, longer than 35 months would allow for all of the retrofits to occur over a period of several years so that facility owners can properly plan outages and suppliers can properly plan for resource availability.