How durable is a silanized surface?
If done properly, reactions with silanes or siloxanes are effectively permanent, unless removed by serious mechanical abrasion (e.g., scraping the material below the coating) or some sort of etching underneath.

The silanization process involves creating new covalent $\ce{Si-O}$ bonds, which are quite strong.


The silane surface itself should be durable. However, the contact angle may change as material adsorbs on top of it. For example, a colleague has shown that hydrocarbon contaminants change the wettability of many surfaces
I don't think any particular storage is necessary. We store in a vacuum desiccator to minimize the effect of contamination, but as long as you clean the surface, the properties should be consistent.