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.