1. When is the library open?

It is open from 8 a.m. to 5 p.m.

2. What time is the library open?
It is open from 8 a.m. to 5 p.m.

3. What are the opening hours of the library?
The opening hours are from 8 a.m. to 5 p.m.

4. What are opening hours of the library?
Opening hours are from 8 a.m. to 5 p.m.

5. What are the opening hours of the library?

It opens at 8 a.m. and closes at 5 p.m.

(Would you check the questions and the answers? Are they all grammatical? Which ones are common expressions?)

Most of these are right, except I wouldn't use #4.

See your other post below for specifics.

The questions and answers are all grammatical. However, the common expressions are numbers 1, 2, 3, and 5. These variations of asking about the opening hours of the library are commonly used in everyday conversation. Number 4 is grammatically correct, but it is less commonly used in spoken English compared to the other variations.