The amount of damage a roof needs to be replaced depends on the type and age of the roof, as well as any existing issues. Generally speaking, if there is significant water damage or missing shingles, it's time for a replacement. If the roof is more than 20 years old and has not been regularly maintained, then it may also need to be replaced. Additionally, if the area experiences extreme weather conditions such as hail or high winds that can cause extensive damage to roofs, then replacement may be necessary even sooner. Ultimately though, only an experienced professional can provide an accurate assessment of whether your roof needs replacing or not.