I think it depends largely on the infrastructure in the area surrounding the affected area. In the U.S. the disaster recovery would be faster and people affected would not need to go so far to get relief. In your example of Nepal, the geography alone is a severe hindrance, not to mention the lack of infrastructure in the unaffected areas.

Haiti recovered faster than Nepal because of proximity to the U.S. Just finding a usable airstrip in Nepal after the quake was difficult, not to mention one that could handle large cargo aircraft. The land routes through the mountains from India were largely blocked and needed to be cleared. Once a support base was established that could bring in large cargo aircraft, the land routes from there to the rest of the country were difficult even before the quake.

So many variables. Geography, infrastructure around the area, the level of destruction, number of casualties, health and sanitary conditions, endemic diseases. All will have an impact on the time to recovery. Even culture can either help or hinder.