makeDelayedFunction.js

function makeDelayedFunction(oldFunction, delay) {
    var obj = { timeout: false };

    return function () {
        if (obj.timeout) {
            clearTimeout(obj.timeout);
        }

        obj.timeout = setTimeout(function () {
            obj.timeout = false;
            oldFunction();
        }, delay);
    }
}

usage.js

var delayedFunction = makeDelayedFunction(function() {
    //$.ajax...
});

$('#text').change(function() {
    delayedFunction();
});

fatto, spero sia comprensibile. È minimale ma vado di fretta