I found this old old study which said that at least after 5 years it is still good, although the peak is after only one year (...)
http://www.ncbi.nlm.nih.gov/m/pubmed/2180995/
Thirty-one men with androgenetic alopecia completed 4 1/2 to 5 years of therapy with 2% and 3% topical minoxidil. Hair regrowth with topical minoxidil tended to peak at 1 year with a slow decline in regrowth over subsequent years. However, at 4 1/2 to 5 years, maintenance of nonvellus hairs beyond that seen at baseline was still evident. Topical minoxidil appears to be effective in helping to maintain nonvellus hair growth in men with androgenetic alopecia.