There are countless styles that a futon frame can be configured in and some of the strongest ones will last for decades if not longer. They key to a strong frame is strong materials and some of the strongest and longest lasting are hardwoods that are common in many different types of furniture.
Although metal is often used when building a futon, it is typically cheap metal tubing which is easier to assemble and sell than a wooden frame. The disadvantage is build quality and these flimsy metal frames have given futons a bad name for years. They are under-engineered to save a few bucks and when you consider how much weight these sofa-beds have to support: 3 full grown males or roughly 1000 lbs of live weight, you begin to understand how important a strong frame is.
A well constructed futon can last for well over 30 years, often longer than the highest quality mattresses, and much longer than the cover. When they are finished correctly and taken care of the frame becomes a piece of art that complements the room as well as preserving its luster for years. Some of William Brouwers original 1982 futon frames are still just as strong and beautiful today as they were 30 years ago.
That's right, the frame for the futon mattress is a relatively new development. Your grandpa might have seen something similar while he was serving in the Pacific theatre during world war two, but it was nothing like the sofa-bed futon gracing college dorm rooms and the mixed use apartments and homes of today.
Some quality frames are made from hardwoods such as solid cherry, solid oak, solid maple and solid bubinga. European beech is one of the strongest materials that can be used but solid wood is not the only requirement for a strong frame. The joints and tenons have to be big and wide or the frame will break where it is connected to the wood. You could have the strongest wood in the world but if it isn't fastened together correctly it is almost useless as a futon, though it would burn for a long time.
Though many hardwoods are fairly expensive, wood such as tulip poplar are strong yet still inexpensive. Don't fall into the trap of cheap futons which fall apart in months instead of years, buy something that is designed to last instead. It will be much cheaper in the long run. There are still inexpensive futons that are not cheap at all and will last for years and years. With a great mattress, futon frame and replaceable covers these functional sofa-beds can last a lifetime.