In AutoCompleteExtender in almost every key press in the textbox, the Ajax call goes to the associated web service. Few service calls are stopped by caching if it is enabled. But in most cases it fires. So, if you enter 20 character, then 20 times call goes to the service and subsequently, 20 times request goes to the database from the service and so on.
If you want to do a more intelligent filtration of the service call, then there is no direct way to do that.
– in your textbox, after certain number of characters you do not need the auto complete suggestion any more.
– you want auto complete suggestion only after a particular pattern of character sequence (e.g. for.., @..) in the text box.