I’ll quote from the National Vaccine Information Center:
According to the CDC, the vaccine reduced shingles by about half (51%) in adults 60 years and older.1 The manufacturer product insert states: “Vaccination with Zostavax does not result in protection of all vaccine recipients. The duration of protection beyond 4 years after vaccination with Zostavax is unknown. The need for revaccination has not been defined.”2 However, efficacy studies showed a significant decrease in vaccine effectiveness one-year post-vaccination and by nine years, Zostavax was determined to be no longer effective at preventing shingles.3
So, sounds to me like they don’t think it lasts very long, and it isn’t clear if getting a new dose later would actually increase effectiveness again.