There’s no universal rule for that. You should make this decision based on your staff’s availability (longer service hours mean higher salaries for customer service agents) and your customers’ activity. For example, if you notice that your followers are most active during “regular” office hours from 9 to 5, it’s logical to provide social media assistance during that time.
However, if your customers prefer to contact your brand via social media in the evening and don’t submit many inquiries in the morning, it would make more sense and be more convenient for your users to provide social media assistance in the evenings.