Hello :),
I think you can do something like that:
unsigned long now = 0;
unsigned long time = 0;
int x = 320; //or whatever is the size of your screen in pixel
int textSize = 150; //or whatever, you have to test for the correct value, or maybe you have a function getTextSize?
void loop()
{
now = millis();
if ( now - time >= 10 ) // 10 milliseconds has passed
{
time = now;
lcd.setStr("ArduinoPat", x, 20, C_COLOR, BACKGROUND);
x--;
if (x < -textSize) //if the text is out of the screen, reset x so the message appear again from the right
x = 320;
}
}