Edit in JSFiddle

var hz,
    period,
    runs = 0,
    startTime = new Date;
do {
  // Code snippet goes here
  runs++;
  totalTime = new Date - startTime;
} while (totalTime < 750);


// period → how long per operation
period = totalTime / runs;

// hz → the number of operations per second
hz = 1 / (period/1000);

// can be shortened to
// hz = (runs * 1000) / totalTime;

document.getElementById("runs").innerHTML = runs;
document.getElementById("period").innerHTML = period;
document.getElementById("hz").innerHTML = hz;

<p>Message for old browsers: do {...} while (totalTime &lt; 750);</p>
<p><span id="runs"></span> cycles run</p>
<p><span id="period"></span> ms for one cycle</p>
<p><span id="hz"></span> hz (calculated cycles per second)</p>