Question
when is the proper time and how often should I spray chemical( as a preventive against bugs,etc.)on my young apple and cherry tree. Thank you.

Answer
This is not an easy question to answer since applying pesticides will depend on the size of the trees and the stage of growth (just budding out to blossoming to fruit present) and what type of insects or diseases are present. Fruit growers usually use a schedule of sprays throughout the growing season. Here is a link to an Extension Service schedule that will detail when and the pesticide to use. This one is from Missouri but I am sure your local state Extension Service has a similar one.
    http://extension.missouri.edu/explorepdf/agguides/hort/g06010.pdf