Every So-many Seconds

int lastTimeIChanged;//in milliseconds (1000 = 1 sec)

int changeInterval = 3000; //how many miiliseconds between each change?

void setup() {

  size(500, 500);

  background(random(255), random(255), random(255));

}

void draw() {

  if (millis() > changeInterval + lastTimeIChanged) {

    background(random(255), random(255), random(255));

    lastTimeIChanged = millis(); //you must remember the last time the sketch changed

  } 

}