According to Bread.com, the molding time of different types of bread depends on many factors. Two important factors that affect molding time are the moisture content of the bread and the presence (or lack thereof) of mold inhibitors or preservatives in the bread.
In a study published by the University of California, Northridge Wonder bread was compared with rye and wheat to see which bread grew mold the fastest. The mold grew slowest with the Wonder bread, while rye and wheat grew mold at a much faster rate. The study speculates that this was due to various preservatives added to Wonder bread.