Because different materials are used for tire tread, and of other variables like rider weight and riding style, it's very difficult to suggest how many km/miles you can expect from a tire.

As a rear tire wears the tread will disappear. It will become obvious. As long as the casing is intact, the sidewalls are not fraying and you're not flatting, you can still ride safely. If you're unsure about your tire wear, ask a knowledgable friend or your LBS to inspect it for you.