A properly installed, well-maintained standing seam metal roof can very well last much more than 50 years; we've seen 100-year-old roofs that are still function perfectly. In that time, you could easily go through five or six standard shingle roofs. There are a few important issues to consider:
1. A metal roof requires different maintenance than an asphalt shingle roof. You need to check it every three or four years, to make sure there are no budding issues with corrosion. Touch up the paint where necessary, and repaint the whole thing in about 20 years.
2. You need to choose a roofing company with lots and lots of experience in doing metal roofs. Ask for references, especially for clients from a decade ago.
3. Metal roofs can be noisy, especially during hailstorms, so plan on adding extra insulation for sound protection.
4. Metal roofs shouldn't be installed on roof slopes flatter than 3 in 12.