Hi jerodw,
You can see the minimum quality/durability guidelines I would generally suggest in post #4 here.
I’m not sure which eluxury mattress you have but if it contains more than “about an inch or so” of 3 lb memory foam in the comfort layers I would generally suggest avoiding it because 3 lb memory foam is a lower quality and less durable material than I would normally consider.
The Tuft & Needle uses 2.9 lb polyfoam in the comfort layers which is a high quality material and has a 1.8 lb polyfoam base layer so there are no lower quality materials or weak links in their mattress relative to more average weight ranges although I would add a slight caution for those that are in higher weight ranges (more than the lower 200’s or so) because the 1.8 lb polyfoam base layer is lower density than I would normally suggest for higher weight ranges.
Yes it would be reasonable to expect that it will soften to some degree but this will happen more over the first 30 - 90 days or so while the mattress is breaking in and then more slowly after that so I would probably wait a little longer before returning it if the comfort level is “close”. There is more about the break in and adjustment period for a new mattress in post # here.
I’m not sure whether you are using a mattress protector or a mattress pad but if you are then depending on the type and thickness it can also have some effect on the feel and firmness of the mattress (a thicker mattress pad or protector can reduce the ability of the foam layers to contour to the shape of your body and can “firm up” the mattress).
It would be reasonable to expect that 3 lb memory foam will soften and break down faster than higher quality/density memory foam or other higher quality foam materials.
While there is no way to quantify how long any mattress will last for a specific person 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 or the useful life of a mattress that really matters) because there are too many unknowns and variables involved that are unique to each person … 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 (see post #2 here) and you have confirmed that it 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 higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer and the chances that you would have additional “bonus time” would be higher as well.
You could certainly expect the Tuft & Needle mattress to be in this range but IMO you would be very fortunate to get 5 good years from a mattress that uses 3 lb memory foam.
Phoenix