By doing calination, almost half of the reaction will be done between the composition. It is important in the Solid State Synthesis of the materials. What is your composition. The calcination is varied for different compositions. Lots of work have done on this. Try to read some literature on this and take further steps to get high density compositions. If you keep the composition long time in the furnace, sometimes the elements will over melt or evaporate. Its depends on the compositions.
This is a very general question. As Venkata stated above, it depends on what you are calcinating. If the reaction is new, or you are trying to calculate a new product the time of calcination can be obtained by experiment and also referring to similar reactions reported in the literature.