I have a cylindrical power of -0.5 (both eyes) with axis 90 on left eyes and 85 on right eyes. Is it necessary to wear my glasses all day?

Probably not. You have a mild astigmatism and at 20 years of age, are probably able to see quite well without glasses. If you get headaches when you read you might consider glasses.
Low power. This is a very small corrective power. Many would not have given glasses at all for a correction this small. It sounds like you have already purchased them - sorry about that in terms of the expense. You will not need them during daylight hours at all. They might provide a little extra clarity at night or when it is raining. You certainly don't need to wear them all the time.