A differentiation has to be made between the initial application (the enterprise registration) and all further applications for other employees.
The enterprise registration is usually valid for a period of five years. Registration validity for smaller companies who invest smaller amounts of capital will often be limited to one or two years.
The validity of E-2 visas for employees of smaller companies often depends on the remaining validity of the enterprise registration. If, for example, a company was registered for five years in 2005 and an employee applies for an E-visa in 2009, the visa will most likely be issued for one year (until the registration validity expires).
For medium-sized companies and large corporations who have a high number of American employees, E-visas will usually be issued for five years.
The E-visa can be extended for a virtually limitless number of times, as long as the company is registered and the employee has an employment contract (with either the American company or the company in the treaty country).