Scotchgard is part of a group of chemical called surfactants, which means that its particles immediately seek the surface of what ever it comes in contact with. These particles help to keep staining substances from binding with the material and creating a permanent stain.
According to the makers, Scotchgard needs to be allowed to dry in a nonaccelerated way (no fans or heaters) for at least overnight or longer. This may not be as problematic when applying Scotchgard to a piece of furniture or a jacket, but carpets -- as one example -- would require you to avoid walking on them for over a day. If the Scotchgard is dried too quickly or applied too thickly, then its layer can become stiff and flaky, requiring you to vacuum the affected area.
Though many options exist for providing stain resistance, such as mill-treatments on carpets, a product like Scotchgard can be easily applied by anyone to most colorfast textiles. This allows everything from furniture to clothing receive a protective, stain-resistant layer, without having to resort to professional services. 3M does not, however, recommend Scotchgard for use on oriental rugs or wool.
Over time, the protective layer formed by a Scotchgard application is bound to wear down. As a result, 3M recommends that a Scotchgard-protected surface be thoroughly cleaned and reapplied at least once a year. However, high traffic areas and frequently-worn clothing should have their coatings updated even more frequently.