Most painted metal siding seems to last at least 20 years. The reason for this is the paint is baked on in a factory giving it a high quality and nice looking surface. This baked on method is used for painting aluminum and most metals siding that gives the product a long life.
I lived in a mobile home park for a few years and my next door neighbor's mobile home had never been painted and was over 45 years old. He would go out and washed the mobile home once a year with some TSP soap while using a brush on the end of an extension pole to clean the home.
His mobile home might not have looked brand new but it was definitely in great shape. Over the years he had developed a few scratches and some dirt that was hard to wash off but that was about the extent of the damage to the painted metal siding.
Well we painted our mobile home and used an expensive metal primer along with some expensive paint from Dunn Edwards & Co... This paint worked great. We lived there for around four years and never had any problems once the home was painted. Having been in the construction business I knew we had to use a good metal paint primer in order for the paint to stick to the baked on paint process for the metal siding to look good.
Here's my advice to you when painting metal siding.
1. Make sure the metal siding is clean. Use TSP soap along with a scrub brush and scrub that baby clean. The cleaner the metal surface the better the new paint primer will stick or adhere to the old paint.
2. Follow the instructions carefully on the paint primer can when applying the primer. Do not paint below or above the recommended paint temperatures.
3. Make sure you explain to the paint salesman at whatever store you're in that you are going to be painting metal siding. Get the right paint primer.