Prenups last, usually by their terms, for the entire length of the marriage. However, prenups sometimes include provisions that expire. The most common one might be an agreement that there’s going to be no spousal support unless they are married for at least 10 years. Many people would think that’s a reasonable compromise. Otherwise, prenups are intended to be valid for the length of the marriage.
Prenups are generally a fairly advanced concept. Most family law attorneys would pretend that they understand what they’re talking about when discussing prenuptial agreements. If you’re seriously considering a prenup or, more importantly, you’re entering a divorce where a prenup was created before marriage, you really need to make sure you talk to company counsel like Holstrom, Block, & Parke.