I don't remember which country it was. I think it was an African country, but I'm not sure. I did find an article [1] about the (failed) eradication of the Aedes aegypti mosquito in Brazil, that contains the following quote:

> In 1973, the Pan American Health Organization again declared that Brazil was Aedes-free. The second official eradication period did not last long. In 1976 and 1977, A. aegypti was found in Rio de Janeiro and Salvador, Bahia, and then in nearly all the Brazilian states.

So, at least three years in their case. Seems you need to eradicate the mosquito from all the neighboring countries as well, otherwise they'll just fly in and fill the void. If you're interested in what they did to declare Brazil Aedes-free (twice), the full article is worth a read:

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5343710/