It depends if the application is horizontal or vertical and what type of exposure the structure is subjected to. Generally speaking, you should expect 8 to 12 years before reapplying. A simple test is to stream water onto the treated surface. If it is still beading water there is no need to reapply.
The Rilem tube is an easy and generally effective test to determine the porosity of a substrate by testing the substrate prior to applying a water repellent and then again after it has been applied. In the case of Shield MX, wait for two weeks to test so that it has fully cured. Mark the areas tested to achieve reproduceable results.
Shield MX is a proprietary silane/siloxane which has a very small molecule resulting in deeper penetration. Shield MX will not etch most glass (always test a sample), will not alter the color or light reflection of the substrate, and can be applied a 40⁰F as long as it has one hour to cure before temperature drops.