Veterans may be entitled to receive up to 36 months of education benefits under this program after completing the full period of enlistment. This applies if veterans were discharged for the convenience of the government after completing 20 months of an enlistment of less than three years, or 30 months of an enlistment of three years or more.
However, veterans who were discharged for other specific reasons before completing their enlistment period (for example a discharge based on a service-connected disability, a medical condition pre-existing service, hardship, or a reduction of force), will earn only one month of entitlement for each month of duty after June 30, 1985, and also earn one month of entitlement for each four months in the Selected Reserve after June 30, 1985.
Veterans may receive a maximum of 48 months of benefits under more than one VA education program. For example, a veteran who used 30 months of Dependents' Educational assistance (chapter 35 of title 38, U. S. Code) and who is eligible under chapter 30, could have a maximum of 18 months of entitlement remaining. A veteran who used 27 months under chapter 34 before December 31, 1989, and who is eligible for chapter 30, could have a maximum of 21 months of entitlement remaining.
https://www.gibill.va.gov/apply-for-benefits/road-map/