Hi davess,
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.
Since you are only in the lower 200’s the foam densities would still be suitable for your weight range and I would only have some concerns with the soft version partly because of the convoluted foam (see post #2 here) and partly because firmness/softness can be a factor in durability and it could also be a more risky choice in terms of suitability and PPP as well.
While it would would be less durable than higher density memory foam (or other more durable types of foam such as higher density polyfoam or latex) … assuming that the medium wouldn’t be close to the edge of being too soft for you (and of course it’s unlikely that the firm would be close to the edge of being too soft) it would be reasonable to expect a useful lifetime in the lower end of the 7 - 10 year range and I would treat any additional time after that as “bonus time”.
Phoenix