Expectations will vary depending on the kind of job being worked, and this ultimately determines how Slack will be used: either like an instant messenger (IM) or like an email client.
In a startup, there’s an expectation to respond during work hours, and with a latent expectation to do so (as needed) during non-work hours. Places like these tend prefer synchronous (replying instantly) over asynchronous (replying later) communication, as it allows decisions to be taken quickly.
In an enterprise, there’s an expectation to respond only during work hours, unless your role explicitly requires it e.g. an executive or an SRE/DevOps role.
Enterprises move much slowly than startups, so they generally skew towards asynchronous communication and tend to use Slack like email (although small teams embedded inside enterprises might use Slack like an IM.)
So, how does your team use Slack? If they use it like email, then you need to have teammates include their working hours on their Slack profile, to allow the rest of the team know when to expect a response.