A prenuptial agreement is a contract and like many contracts, its lifespan depends on the terms included in it. Before the wedding, future spouses can decide in a prenup who is going to pay for what during the marriage. They can exempt property from their state's distribution laws in the event of divorce and plan how they're going to handle their estates when they die. How long the agreement lasts is entirely a matter of personal choice.
Prenups Are Forever...
If a prenuptial agreement is silent regarding how long future spouses intend for it to remain in effect, it exists forever. No state law dictates a duration for such contracts. Its life is indefinite, but this doesn't mean you and your spouse can't kill it. You can revoke it at any time by superseding it with a secondary written agreement, stating that the first is no longer in effect.
Unless They're Not
Some prenups include sunset clauses -- agreed times at which they cease to be valid. You and your future spouse might agree that your prenuptial agreement only remains in force during the first 10 or 20 years of your marriage. Another option is to include terms that dictate that your separate property becomes marital property after a certain period of time.