As I know it is 9 am to 6 pm and during the weekend it is extended from 8 pm to 11 pm. Take note that Night Safari is not everyday but generally only on Fridays and Saturdays and sometimes on the even of public holidays/school holidays.

My suggestion is that you check and confirm the Night Safari is open on the date you are planning to go.

Though I have not been there myself, from what I heard from my friends and relatives, during Night Safari you can walk around freely. However do take note that the Zoo is huge. I also heard that the Night Safari is not worth the visit and not that interesting.

When I visited the Zoo recently I felt it not as well maintained or impressive as was during my previous visit 5 or 6 years ago.