Mo. Rev. Stat. § 429.170: Actions commenced in six months.

All actions under sections 429.010 to 429.340 shall be commenced within six months after filing the lien, and prosecuted without unnecessary delay to final judgment; and no lien shall continue to exist by virtue of the provisions of said sections, for more than six months after the lien shall be filed, unless within that time an action shall be instituted thereon, as herein prescribed.