How to understand that the client waits too long for an answer?

To prevent clients from waiting too long, you can set the desired response time in Settings > General > Long Response Notification. When over 80% of this time expires, the chat will be marked yellow. When the time limit expires, the chat will be marked red. So you will always see which chats require the biggest attention. 

An example of how an expired chat is displayed. The chat has a red mark.