When should I move a task to Waiting on Kanban board?

Kanban is an incremental, evolutionary process based on a constant improvement approach, called Kaizen. Use of the Waiting column depends on your workflow, capacity, and the WIP limits applied to the “In progress” column. Here are the 2 most common scenarios in which a Waiting column is used:

• You should move tasks to Waiting when you are ready to be pulling new tasks. When planning your work for a day, or a week, place as many tasks as you should be able to complete within this time in the Waiting column. It will create a buffer with all your planned tasks, from which you’ll be able to pull after completing previous work.

• You should move a task to Waiting when it’s waiting for somebody else’s approval, or more information is needed to complete it.
  In multi-leveled teamwork, it’s a common occurrence for senior team members to delegate tasks to the junior team. These tasks then may require approval or comments from the assigning personnel. I.e.: you’ve completed the work for now, but it needs approval - so you move the task to Waiting. Once approved, it will either be moved back to Doing for you to change something, or it’ll be moved to Done.

Related: Remember to set a WIP limit on your Working column.
You can find more information about WIP limits and Kanban boards here: Importance of Kanban WIP limits and Kanban board.

There are many more possible uses for a Waiting column, feel free to use a solution that works best for you and your team.