My shortbread can be a bit hit and miss...but as with all home baking, things have to be pretty bad before no one eats it.
I use Delia's recipe, but instead of rolling it out into a circle, I roll it into a long sausage shape , chill it in the fridge for about half an hour then slice it into rounds. (If I have the time I also sometimes press some demerara sugar into the sides of the sausage before slicing).
Then you bake them for about 15 minutes....but you have to keep an eye on them as the time they take depends on the thickness of your slices. Also depends on how brown you like your shortie. We tend to prefer it really pale, so I get it out sharpish....also you can always put it back in if you think you've undercooked it, not so easy to get it back if you've overbrowned it...still , as I said...things have to be pretty bad before we don't eat them.