Sometimes roofs age naturally and reach their end without showing signs of collapsing. If it looks worn, then doing preventive maintenance is normal. However, if roof replacement is delayed, bigger problems might occur. So, make sure you watch for any warning signs, be sure to give plenty of time, and do not wait until the very last minute.
Below you will find plenty of information and advice on how and when you should replace the roof, but as a rule of thumb, 20 years should be the minimum. In the case of stronger materials, you might even be able to wait 50 years.