My MacBook Air is currently loaded with OSX Lion. How do I load El Capitan? I do not have an internet connection that will stay connected for days
How to download El Capitan to my MacBook Air
MacBook, OS X Mountain Lion (10.8.5)
This explains how to upgrade:
Upgrade to OS X El Capitan - Apple Support
The download required is huge (about 6 gigabytes, as I recall). So the length of time required to download the software depends on the speed of your internet connection.