Help | Min and Max Schedule
What is the maximum and minimum amount of time I can schedule messages
The shortest time you can schedule a message in Slack with the /schedule command is 30 seconds.
The longest time you can schedule a message is 120 days or equivalent. Ie roughly 4 months. This limit will also apply to recurring messages. If you are scheduling a message to repeat every month, it can only repeat until it hits the 120 day limit.