Benefits end 10 years from the date of a veteran's last discharge or release from active duty. VA can extend that 10-year period by the amount of time a veteran were prevented from training during the period due to a disability or being held by a foreign government or power.
If a discharge is upgraded by the military, the 10-year period begins on the date of the upgrade.
Veterans who are eligible based upon two years of active duty and four years in the Selected Reserve have 10 years from release from active duty, or 10 years from the completion of the four-year Selected Reserves obligation to use benefits, whichever is later.
https://www.tcc.fl.edu/student-life/veterans-affairs/guidance-for-choosing-gi-bill-programs/