Hi kawkaw2,
There is no way to quantify how long any mattress will last or predict exactly when you will decide to replace it because it is no longer suitable or comfortable for you (because this is the only real measure of durability that counts) because there are too many unknowns and variables involved that are unique to each person but if a mattress is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new (which is unlikely with the Tuft & Needle) and meets the minimum quality/durability specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with materials that are higher quality and more durable than the minimum guidelines (in the top comfort layers especially) it would likely be in the higher end of the range or even longer.
There is also more detailed information about all the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to but since the Tuft & Needle meets all the minimum guidelines and there are no lower quality materials or weak links in the design … as long as you aren’t in an unusually heavy weight range (mid 200’s or higher) then it would be very reasonable to expect that it would maintain it’s comfort and support for most people for 7 - 10 years and in some cases longer.
There is more information about edge support with foam mattresses in post #33 here but it certainly wouldn’t be an issue for most people unless you often sleep or sit with most of your weight concentrated on the very outside edge of the mattress although the outside edges of a foam mattress would generally be softer than an innerspring mattress that has a specific edge support system.
The only way to know for certain whether the edges of the mattress will be suitable for your specific body type and sleeping style will be to actually sleep on it just in case you are one of the few (generally in higher weight ranges) that needs or prefers a firmer edge but they also have a great return policy so if for some reason you aren’t happy with the edges of your mattress (either for sitting or sleeping) or for any other reason then there is no cost involved in returning it.
Phoenix