Masterwork Research: After finishing the Pokémon GO Tour: Kanto Special Research, you’ll be granted access to a new, more challenging type of research: Masterwork Research! Masterwork Research is designed to be completed over a long period of time, and the first story—All-in-One #151—will result in an encounter with Shiny Mew!
Do I need to finish the Special Research or Masterwork Research by a deadline to encounter Shiny Mew?
No, there is no deadline to complete either the GO Tour: Kanto Special Research or the Masterwork Research acquired after completing the Special Research. Masterwork Research is meant to be completed over a long period of time, so take as long as you need.
Complete the Pokémon GO Tour: Kanto Special Research to unlock Masterwork Research—and a chance to encounter Shiny Mew!
Work alongside Professor Willow in Pokémon GO Tour: Kanto’s Special Research story, where you’ll learn more about the Pokémon originally discovered in the Kanto region, practice your Trainer skills, and come across some clues related to a mysterious Pokémon. At the end of the Special Research, you’ll encounter a Shiny Ditto!
After completing the Pokémon GO Tour: Kanto Special Research, you’ll unlock a new kind of research called Masterwork Research, which will take determination and skill to complete. Just completing the first page will require that you receive your platinum Kanto medal. From there, you’ll have to reach level 40, catch 30 Pokémon of each type, spin 151 unique PokéStops, walk 151 km, and more—an incredibly special encounter with Shiny Mew lies at the end of this adventure!
Source: Official Pokémon GO blog