Hi tabdams4,
There isn’t a way to quantify how long a mattress will last for any particular person because it depends on many things including where they are in in the “I can sleep on anything” to “princess and the pea” range and when they cross the thresholds in between sleeping well on a mattress to sleeping “OK” to tolerating a mattress to finally deciding to replace it. Having said that … with 3" of 3 lb memory foam I don’t think it would be reasonable to “expect” this mattress to last for 5 years no and the reason I suggest avoiding 3 lb memory foam (see the guidelines here) unless you are in the very lowest budget ranges and there are no other better options available is because the risk is too high that it may last significantly less than that.
IMO … there are better quality/value options included in the list of the better online memory foam manufacturers and retailers linked in the mattress shopping tutorial (in the optional online step).
The better options and possibilities I’m aware of in and around the Northern Virginia area (subject to the guidelines here) are listed in post #2 here.
Phoenix