Adam Clarke on Genesis 29:27

Fulfill her week - The marriage feast, it appears, lasted seven days; it would not therefore have been proper to break off the solemnities to which all the men of the place had been invited, Gen 29:22, and probably Laban wished to keep his fraud from the public eye; therefore he informs Jacob that if he will fulfill the marriage week for Leah, he will give him Rachel at the end of it, on condition of his serving seven other years. To this the necessity of the case caused Jacob to agree; and thus Laban had fourteen years' service instead of seven: for it is not likely that Jacob would have served even seven days for Leah, as his affection was wholly set on Rachel, the wife of his own choice. By this stratagem Laban gained a settlement for both his daughters. What a man soweth, that shall he reap. Jacob had before practiced deceit, and is now deceived; and Laban, the instrument of it, was afterwards deceived himself.