Hi FGCU,
The best place to start your mattress research is post #1 here which has all the basic information, steps, and guidelines that can help you make the best possible choices. It will also help you avoid the worst ones which includes the major brands and the chain stores and other mass market outlets that know very little about the mattresses they carry and in most cases aren’t able to tell you the specifics of what is inside them (a mattress is only as good as its construction and the quality of the materials inside it).
You can see some of my thoughts about buying from Costco or other big box stores in post #4 here.
The durability and useful life of a mattress includes many factors including the design of the mattress, the quality of the materials, and its suitability for a specific person in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). There is more about durability and the useful life of a mattress in post #4 here but durability and the loss of comfort and support that comes from changes in softness or the breakdown of lower quality materials is relative to how much it affects each person and there isn’t a specific amount of time a mattress will last because it depends on both the mattress and on the person. The only “rule” that applies to durability is that higher quality more durable materials materials will last longer than lower quality materials.
Dreamfoam and it’s sister company Brooklyn Bedding is one of the members of this site which means I think very highly of their quality, value, and service and they compete very well with the best in the industry. You can see the Dreamfoam warranty here.
I would make sure that you have a more detailed conversation with them (or any online retailer or manufacturer you are considering) to make sure that the mattress you are considering is the one that has the best odds of being suitable for you both in terms of PPP and in terms of durability. In many cases they can also tell you about mattresses that are available locally that are similar to some of the mattresses they make so you can do some local testing with mattresses that have a similar feel to get a better sense of whether they are suitable for you.
It’s always much easier to connect with the experts that already know what you would otherwise have to learn than to try and become one yourself. I would also keep in mind that mattress reviews only apply to the person that writes the review because each person has very different needs and preferences and what works well for one person may not work at all for the next (which is one of the reasons why a more detailed conversation with a knowledgeable online retailer or manufacturer is so important).
Phoenix