When you select "Place caller in wait queue" as the fallback action for any rule or condition in a call queue, the wait queue setting will be enabled for that call queue. Here, you can customize the duration (in seconds) for which you want to place the callers in the wait queue and also take appropriate action if the configured maximum time exceeds. 

The default maximum queue time is 900 seconds or 15 minutes. For more details, see Configuring Wait Queues.