There are no hard and fast rules, but generally, the sooner the better. Ideally, you want to start the process at least 4 months before the wedding date. That would allow up to 3 months to prepare and finalize the agreement, with one clear month between signing the agreement and your wedding date. The idea behind the clear month is so that there is no pressure on one party to sign the agreement, and there is time after the agreement to change one’s mind if one regrets signing it.
If your financial or legal situation is more complicated, then you would want to start sooner, and if your situation is not as complicated, you may be able to start later.
If the timing is tight between the agreement and your wedding date, consult with a lawyer as to whether it is appropriate to proceed with the agreement. There are also other options you should consider. The first is postponing your wedding date. The second is what is known as a “stand still” agreement. This is a prenup that basically states you and your partner intended to enter into a prenup prior to your wedding, but were unable to do so due to time constraints. The two of you plan on entering into a prenup after the wedding. The agreement states that if the marriage ends, then you and your partner are to be in about the same position as if the marriage had not occurred. The stand still agreement expires after a time period (typically 6 months to one year), which is the deadline for you and your partner to enter into a properly negotiated prenuptial agreement.