Debounce the textarea change.

Demo: jsFiddle

Put your ajax call in the saveToDB() function. These event names('input propertychange change') will trigger on any form element change such as radio buttons, inputs, etc.

var timeoutId;
$('#the-textarea').on('input propertychange change', function() {
    console.log('Textarea Change');

    clearTimeout(timeoutId);
    timeoutId = setTimeout(function() {
        // Runs 1 second (1000 ms) after the last change    
        saveToDB();
    }, 1000);
});

function saveToDB()
{
    console.log('Saving to the db');    
}

Here is a full demo showing you how to debounce a full form and use ajax to send the data and then return the status (Saving, Saved, etc).

Demo full form and ajax: jsFiddle