You can expect a metal roof to last at least 2 to 3 times longer than a regular roof. In general terms, count on a metal roof lasting 40 to 60 years and beyond.
To put it in context, the average life span of an asphalt roof is 12 to 20 years. That lifespan can be shorter depending on the pitch of your roof and the climate in your area. Made of oil impregnated paper or fiberglass, asphalt begins to deteriorate as soon as you expose it to normal weather. A metal roof, however, will never decompose.
Other roofing materials like wood shingle, shake and tile have varying degrees of weather-related problems that lead to breakdown. Wood shingle and shake roofs often need replacement before twenty years. Concrete tile roofs can crack and warp in the freeze/thaw cycle of more northern climates.
All of the above roofing materials are well-outlasted by metal roofing, which retains its good looks and durability decade after decade after decade.