Hi disalex,
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 they meet the minimum quality/durability specs relative to your body type and weight that are suggested in the quality/guidelines 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.
There is more about the many variables that can affect the durability and useful life of a mattress in post #4 here.
Some of the information in post #4 here may be helpful as well.
Phoenix