Time - final decree of divorce

Apply for certified copy of order and decree of divorce at the copying branch of the district court where the divorce case decided. Within a week's time you'll get it.

As per my knowledge

There is no expiration date on papers that have not been filed with the court - the court can dismiss the case after it is filed and nothing is done to move it along.

However, if you signed a separation agreement & then move back in together, the agreement is no longer valid unless you both reaffirm it. If you remain separated, the separation agreement remains valid. You can file for divorce using it any time.