New to cooking - how long to cook chicken breast pieces in a wok?

just wondering, how long should I cook chicken breast pieces in a wok over medium heat for? They're around 1cm thick chicken breast pieces.

I tried cooking them today and although they looked white, one piece had a tiny streak of blood (blood vessels?). I was kinda scared so I didn't eat it although most of the chicken looked fine apart from that one piece with blood. And I mean it was surrounded by white

So does that mean I undercooked them? How long should I cook them for? Medium heat is around mark 4/6 on the hob. All the pieces looked white and not pink but I was just concerned

Thanks