Solely based on this article, I’d suggest around 8 months after second dose:
the antibodies generated by the Moderna vaccine against the original strain dropped to low or undetectable levels six to eight months after the second dose
Worth noting that Dr. Luis Marcelo Martínez has claimed that “the spike protein is nothing more than graphene inside your body and spiking you”, so it could be that 6-8months is the time it takes the body to naturally detox itself of graphene oxide…