time span 
noun
A period of time between fixed points or marked by the continuation of a particular process: the time span of one human life...
- Second, Taiwan stretched its preparatory period over a longer time span than did China.
- Such a large time span meant that a number of styles developed within Gothic architecture and it is common to divide these styles into three sections.
- In building scenarios, the group used a time span of 20 years, but some of the scenarios are more futuristic than others.