Probably 8 to 12 years if the name looks believeable (generic dot com), meaning that it looks like a true generic that probably receives traffic from typeins and probably won't stop receiving traffic within that period of time.
If it's getting typo or trademark traffic, it's not a dot com, or it just doesn't look good, then probably 2 years or less (maybe only a few months).
Also, if it is getting traffic from existing links on the Internet, then it falls under the second category.