A little confusion exists. If you have the fire clay mortar, it is already mixed. If you have the fire clay cement, the mix is as follows:
1 part cement 1/4 part hydrated lime 3 parts sand
The joints for the firebrick should NEVER exceed 1/4". Don't forget to slope the back wall of the fireplace as the original or you will loose the necessary draft. The drying time is dependent on too many factors to give you an accurate count.