Contracts don't typically expire unless an expiration date is included as a provision, or the law imposes a statute of limitations. Separation agreements are enforceable as contracts even if you don't ultimately divorce. There's typically no statute of limitations, so if you've signed one, you may be stuck with its terms.
Separation
A signed agreement can govern the terms of your separation indefinitely. You don't have to get divorced; the terms of the agreement will continue to prevail nonetheless. In fact, as a binding contract, your agreement can remain enforceable even after you divorce. However, your divorce decree can include language stating that it supersedes the terms of your agreement.
Conversion to Divorce
Assuming you have no problem with your separation agreement, you can merge it with your decree if you later decide to divorce. In some states, this is as simple as submitting a copy to the court along with your divorce paperwork and asking that its terms be incorporated in your decree. Divorce decrees don't expire or have a statute of limitations either.
Modifications
The terms of your agreement remain binding, either as a stand-alone contract or merged into your decree, unless either you or your spouse goes back to court to modify them. Depending on the wording in your agreement, you might not be able to change the terms regarding alimony or property division, but custody and child support provisions can always be altered later if there's a change of circumstances.