Rashi on Genesis
שתים עשרה שנה עבדו TWELVE YEARS DID THEY i. e. these five kings SERVE Chedorlaomer.
Radak on Genesis
שתים עשרה שנה עבדו, they were enslaved to Amrafel, paying annual taxes to him.
Sforno on Genesis
עבדו את כדרלעמר, after the compromise they had reached 12 years ago. Afterwards, they rebelled.