Just finished sowing my first early potatoes what is the general rule of thumb that the shoots will start to appear on the surface and when should I start earthing up

The shoots will appear as they are ready to and you earth them up as soon as they appear to the point that they disappear. Keep doing this until you can't possibly earth up any more!

I have put mine 3 weeks ago and the shoots are still really small, like 5mm or so, they don't seem to grow at all. Someone told me perhaps it's too early.

I agree with Mg above. If you expect frost earth up any shoots that are showing.

Not too early Prince and I assume you mean 5 cm as it would be difficult to see growth of 5mm. Earth them up and they will keep growing onwards and upwards.