The basic answer is... well, you make sure to load it in time before it has to be shown.
Rather useless answer, but without further specification its hard to answer better.
If you want to repeat the same, then you can draw the same model twice without having to reload anything.
Though, in practice, this gets a lot more complicated.
When dealing with huge meshes like landscapes, you also need to make sure to handle "level of detail" (LOD), to make sure to not draw billions of barely visible micro triangles wich kill your performance.
Making sure to load in and out parts of the landscape as needed usually ties into the LOD algorithm.
Its far from a "basic" problem... But you could start with researching what LOD is and why it is needed.
The most basic algorithm to check if something is close enough it has to be loaded and/or drawn is simply checking the distance.
Another nice structure to speed this up is Quad trees.
Edited by Olof Hedman, 05 May 2014 - 07:14 AM.