We know from the title of chapter 29 that five years after the end of the game, Turtle is eighteen and in her second year of college. She has been regularly visiting Westing ("the master") and won her first chess game against him. This could be foreshadowing Westing's declining health.
The exact amount of time that has passed between the end of the game and Westing's death, however, is not known. We know that Westing is eighty five years old when he dies, but we do not know how old he was when the game began. During the game, Sandy McSouthers (who is Westing in disguise), in his own notes claims to be 65 years old. Crow, Westing's ex-wife, was 57. It is likely that she and Westing were somewhat close in age.
Combine this information with context clues which suggest a signficant amount of time has passed between the end of the game and the final scene. Crow and Otis are dead, Angela is an orthopedic surgeon and has a ten year old daughter, Chris is a college professor, and Doug has won two olympic gold medals in a row. Turtle has worked in various positions for Westing, but has most recently been his legal confidant.
Given the combination of all of these context clues, it is reasonable to say that at least 20 years have passed between the end of the game and Westing's death.