You can display a message when a customer's question is too long and hence it is unlikely to receive a relevant answer.
To define a message, do the following:
Let's say you don't want your customers to have questions with more than 40 characters and when they type 30 characters, you also want to let them know that their question is getting too long.
In this case, you would set the Maximum length of user queries value to 40 as this prevents customers from typing more than 40 characters. At the same time, you would set Query length for displaying warning to 29 so that when this threshold is exceeded (when the customer types the 30th character), a warning is displayed.
Notice that if you want to display a warning exactly when the maximum length is reached, you would have to set the value of the Query length for displaying warning to one character less than the maximum length defined. If you set both values to the same value then the query length threshold will never be exceeded and, therefore, will never be displayed.