Most of today’s asphalt shingles are designed to provide adequate service for a period ranging from 20 to 30 years or longer. These type of shingles are typically referred to as a 20 year, 30 year, 40 year, etc. shingle. The number of years serves as more of a general grade rating than a definite shingle lifetime. Typically, a shingle should and will need to be replaced prior to its pre-determined life span.
High heat and humidity will definitely lower the life span of any shingle. On the whole, though, you can expect a 30 year shingle to outlast a 20 year, as a 40 year would a 30 year, despite the region your live in. A good rule of thumb . . . start exploring your options now, don’t wait until your ceiling is saturated with water!