If you EAS/retire on April 15, your effective date is April 16. VA pay entitlement starts on the first day of the next month, so May 1. VA pay is in arrears, so first payment would be June 1. That's how you get the roughly 45 days. If EAS was April 1, payment would still be June 1 which would be 60 days after EAS. If EAS was April 30, payment would also still work out to June 1 which would be 30 days after EAS. 30-60 days is the window (assuming DD-214 is uploaded to VA.gov and verified quickly), so 45 days is usually a good estimate.