Preparation time is the time given or needed to prepare classes.
The amount of time needed to prepare a class will vary widely depending on the objective of the class, the teacher's experience, the materials available and the methodology being used. The time available will depend on time management skills and teacher workload. If time available is greater than time needed, then class can be adequately prepared. The problem comes when time available is less than time needed, in which case corner-cutting will need to be employed.
If a teacher is going to create a topical class from scratch then preparation time could conceivably be measured in hours.
If the class is based on a course book then all the materials are provided for the teacher and less work and imagination are needed.
If a teacher is a devotee of dogme then preparation may be minimal.