A 'thousand' means much, many, a countless number, a whole era of time, an abundance. When used in reference to the Lord, it actually means a number greater than that: In those cases it means what is infinite, what is truly without end.

A 'month,' as in Genesis 29, signifies the end of an earlier state and the beginning of a following state, thus a new state. 'A month' signifies a full state.

(References: Arcana Coelestia 3814)