declare("MyFilteringSelect",FilteringSelect,{
				// custom min input character count to trigger search
				minKeyCount: 3,// override search method,count the input length
				_startSearch: function (/*String*/key) {
				    if (!key || key.length < this.minKeyCount) {
				    	this.closeDropDown();
				    	return;
				    }
				    this.inherited(arguments);
				}
			});

原文参考http://stackoverflow.com/questions/15949425/dijit-filteringselect-with-min-length

searchDelay 
Delay in milliseconds between when user types something and we start searching based on that value