I cannot think of a good way of doing it in Processing JS (in JS setInterval can be used to loop every n second/s), but this code should work OK:

var x = 0;
var y = 0;

draw = function() {
    background(255);

    ellipse(x, y, 50, 50);

    if (frameCount % 10 === 1) {
        for (var i = 0; i < 20; i += 1) {
            x = random(0, 400);
            y = random(0, 400);
        }
    }
};

Run the change every time floor(millis()/1000) reaches a new integer.