The dental stone model should be left undisturbed for ____________ until the material has set completely.
If the inversion method is used, a mound of stone about 20 mm high is made on a flat surface, such as glass plate, with the remainder of the mixture or with a new mix using a lower W/P ratio. The filled impression, which should exhibit the initial set but not the final set, is then inverted over the mound of stone and the base is shaped with the spatula before the stone sets. The dental stone model should be left undisturbed for 45 to 60 min until the material has set completely. The dental stone model is now separated from the impression. When tooth preparations are involved, a smaller spatula may be used to deliver a much smaller amount of stone to fill the sites of the preparations and ensure complete filling before the remaining portion of the impression is poured. 
Phillips Science of Dental Materials ed 12 pg 192
20 min
10-15 min
35-40 min
45-60 min