The length of time that your U.S. Entry Waiver is valid for is determined entirely by Customs & Border Protection, a division of the U.S. Department of Homeland Security. It is not uncommon for first time applicants to be granted a Waiver for 1 or 2 years, but many applicants receive a 2-5 year term immediately (5 years is the maximum). If you are granted a shorter term than 5-years, this gives the U.S. the opportunity to “test” out your conduct during your time in the United States before committing to a longer term.
Regardless of the term of your Waiver, you will be establishing a positive relationship with the U.S. by demonstrating compliant conduct while in the States. If you continue to apply for and get Waivers, without any new problems arising, subsequent Waivers will likely be granted for longer periods of time as you establish trust. Ultimately, your goal is to establish an extended, trouble-free travel history with Customs & Border Protection.