One way to decide when it's time to replace a tyre is to look for the wear indicator. Most road bike tyres will have a small circular depression in the tread. When the surface of a tyre becomes flush with the depression it's time to replace the tyre.
If you can't find the wear indicator there are other ways to tell. Often the best indicator is that you've started to get flats. As a tyre wears the outer surface gets thinner and flats happen more often. You'll also start to develop a squared off profile. If you haven't used a bike for a while then you'll want to look for cracking and rotting sidewalls. Even if there's not much wear, those are signs to replace the tyre.