One simple solution is to use a counter to check how long the key has been held down:

Code:

int laneNum = 3;
int hitCounter = 0;
int hitDelay = 90;

void setup() {
    
}

void draw() {
  if(keyPressed) {
    if (key =='w' && laneNum > 1 && hitCounter <= 0) {
        laneNum = laneNum - 1;
        hitCounter = hitDelay;
        println(laneNum);
    }
    else {
      hitCounter --;  
    }

    if (key =='s' && laneNum < 3 && hitCounter <= 0) {
        laneNum = laneNum +1 ;
        hitCounter = hitDelay;
        println(laneNum);
    }
    else {
        hitCounter --;
    }
  }
}

void keyReleased() {
    hitCounter = 0;
} 

This approach means a quick tap and release is registered as a hit (and this applies to subsequent taps because hitCounter is set to 0 when the key is released).  If you press and hold the key, hitCounter is set to hitDelay (adjust this value to determine the pause between hits - higher numbers equal longer pauses) and reduced by 1 each frame until it reaches 0 and a hit is registered.

If you want to allow multiple keypresses (e.g. you want the user to be able to move left and right as well as up and down all at the same time), you may want to look at this thread to see how best to handle multiple keypresses, though in this case you may need to create a separate counter for each key to deal with what happens when a key is released...