I just went there last month with my family. Open daily from Monday to Sunday, including public holidays.
If you want to come close with the wild Orang Utan, then i suggest you arrive before the feeding time which is 9am - 10am and 3pm - 3.30pm. The best time would be the morning session as the temperature is not that warm if compare to the afternoon session.
The ticket is less than RM5.00 for an adult and it is cheaper for senior citizen and students.