9 The sentence simply begins with “twelve years”; it serves as an adverbial accusative giving the duration of their bondage.
10 This is another adverbial accusative of time.
11 The story serves as a foreshadowing of the plight of the kingdom of Israel later. Eastern powers came and forced the western kingdoms into submission. Each year, then, they would send tribute east—to keep them away. Here, in the thirteenth year, they refused to send the tribute (just as later Hezekiah rebelled against Assyria). And so in the fourteenth year the eastern powers came to put them down again. This account from Abram’s life taught future generations that God can give victory over such threats—that people did not have to live in servitude to tyrants from the east.

In other versions

Twelve years they had served Chedorlaomer, but in the thirteenth year they rebelled.

For twelve years they had been subject to Kedorlaomer, but in the thirteenth year they rebelled.

They were subject to Chedorlaomer for twelve years, but in the thirteenth year they rebelled.

For twelve years they had been subject to King Kedorlaomer, but in the thirteenth year they rebelled against him.

Twelve years they had served Kedorlaomer, but in the thirteenth year they rebelled.

Twelve years they served Chedorlaomer, and in the thirteenth year they rebelled.

Public Domain