There are no absolutes and quite a few variables in food dehydration.  The only way to become proficient is to dry, dry, and dry some more.  Certain varieties of produce and humidity in the air make a difference in the drying time and quality of dried products.

Experiment with different drying temperatures, thicknesses of produce, and

pre-treatment vs. no pre-treatment.

Generally fruits should be dried at 130° to 140°, (135° work well).  Vegetables should generally be dried at 125°.  By drying foods in this temperature range you will minimize the loss of heat-sensitive vitamins A & C.

For jerky start at 160° and half way through turn it to 140°.

Nuts and seeds are high in oil.  If higher temperatures are used, they will tend to become rancid, developing off flavors.  The best drying temperature for them is between 90° to 100°. 

Herbs and spices are most flavorful when they first open and should be harvested while very fresh.  Because the aromatic oils are very sensitive temperatures should be between 90° to 100°.  Herbs generally dry in a few hours.  Do not load the trays too heavily or drying time will be prolonged.  Flowers should also be dried at that same temperature range to help maintain aroma and colors. 

Use these guidelines for the foundation of your dehydrating techniques.  Expand on them as you gain skill and confidence with the experience.