You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Russian_language

Context:
Among the first to study Russian dialects was Lomonosov in the 18th century. In the 19th, Vladimir Dal compiled the first dictionary that included dialectal vocabulary. Detailed mapping of Russian dialects began at the turn of the 20th century. In modern times, the monumental Dialectological Atlas of the Russian Language (Диалектологический атлас русского языка [dʲɪɐˌlʲɛktəlɐˈɡʲitɕɪskʲɪj ˈatləs ˈruskəvə jɪzɨˈka]), was published in three folio volumes 1986–1989, after four decades of preparatory work.

Question:
How long did it take to compile the Dialectological Atlas of the Russian Language?

four decades