Embalming someone can preserve the body for years. People have been exhumed after years and still are in pretty good condition with some decomposing. Embalming is not going to keep you forever and even within the first week can show signs of changes from decomposition. It is a slow process from there. A fact that most people don't know, is that you do not have to embalm someone if you don't want to. I will be a quick burial or a cremation, but some would prefer that instead of being embalmed.
Embalming can temporarily preserve a body, but eventually the body will decompose. The amount of time embalming can preserve a body depends on the quality of embalming, the cause of death of the person and the location where the body will be buried.