Tyres? How often do you change yours?

So generic quetion I suppose but I'm still interested in other peoples opinion on this.

I drive an old (15yrs) Ford Focus, almost exclusively on small country roads, roughly 360km/week. Commuting to and from work, pretty normal use.

How often would you say you'd need to get new tyres? I just feel like mine wears down way to quickly but I don't have much to compare to.