Questions about setup, billing, or a real security concern on a site you're monitoring — send a message and expect a reply within one business day. For an active, in-progress compromise, say so in the subject line.
General questions: within 1 business day. Security-related reports from an active monitored site: prioritized immediately.
If a site you already monitor has an open alert, acknowledging or resolving it directly from the dashboard is faster than email — the full evidence trail is already attached to it.