This all relies on how much wear one considers acceptable. I downgrade shirts as they wear, giving them a vast lifespan, so I have no pat answer.
Polyco blends tend to pill, but otherwise would theoretically be more durable.
That wrinkle free crap is a total killer because edges and hems fray way more readily. The fabric that naturally could bend now ends up breaking.

 

What is the lifespan of a cotton polyester blend dress shirt? What have been your experiences?

Ive read about the 35-50 washes average for an all-cotton dress shirt and 150+ for Mercer shirts, but what about cotton blends?

And what about cotton blend Oxford shirts?

And any difference if the cotton blends are wrinkle free?

Your questions assume that all shirts within a given category will behave the same, and they don't. That's dependent upon other factors as well.

But in general:

1. In the 50's, '60 and the early '70's I wore cotton/poly shirts almost exclusively. (As did virtually everyone else.) The main bugaboo of cotton/poly is that it tends not to wear as well as all-cotton, all other factors being equal, but they rarely are. It abrades much more easily and therefore does not wear well. And after not much laundering, it begins to pill. I never tried to quantify the number of washes which varies depending on the quality of the poly (Some is better, some is worse.) and of the cloth itself, and also its weight and how it's woven. The pills can be shaved but it's a case of diminishing returns before you're left with tissue. The benefits of cotton/poly are that contrary to popular opinion it wicks wonderfully, and therefore breathes very well. Better than all cotton. And of course it dries quickly for the same reason.

Many advances have been made in fabric technology since the hey-day of the cotton/poly shirt. Poly in many forms finds its way into high performance technical clothing. But unfortunately the everyday cotton/poly blends I've encountered are very poor, junk essentially, and don't compare with what once was ubiquitous.

2. The yarn for oxford cloth often tends to be spun more loosely, and the weave can be also. My experience has been that cotton/poly oxford cloth wears particularly quickly.

3. Cotton/poly blends are inherently wrinkle resistant, and therefore don't need to be treated, and aren't. I know it's popular to put your nose up to wrinkle resistant treated all-cotton shirts. But really their only draw backs are a lack of breathability and stiffness. But it's been my experience, that these problems among higher quality shirts are slight, and often not noticeable at all. They deliver wrinkle resistant qualities very similar to the old higher quality cotton/poly shirts, but offer much better durability, and no pilling.

 

That wrinkle free crap is a total killer because edges and hems fray way more readily. The fabric that naturally could bend now ends up breaking.

I have many wrinkle resistant all-cotton shirts and I have never experienced that with any shirt. If anything, I find them more durable than those that aren't.

 

^^+1.
I've found this to be particularly noticeable in the area of collar points. While the collar points on my BB must-be-ironed shirts occasionally fray, the collar points on the wrinkle resistant shirts have never frayed, in my experience. Could that be the result of having to work over the collars of the must-be-ironed shirts with an iron to get that crisp wrinkle free look? :icon_scratch:

 

It certainly must contribute, but also I suspect the wash or coating that permeates the yarn conveys additional strength to each individual thread.

 

My experience, admittedly limited and dated, is that the polymer finishes create brittle fibers that lead to clean break at creased bends instead of the gradual even wear that occurs on an untreated fiber.

 

My experience, admittedly limited and dated, is that the polymer finishes create brittle fibers that lead to clean break at creased bends instead of the gradual even wear that occurs on an untreated fiber.

I don't doubt the factualness of your statement, it's just that I've never experienced it. I know there are different processes, and I know that the breathability and stiffness differs between makes, which I strongly suspect means that different processes produce more or less of these effects. And I would suspect that the process used might also determine whether a particular garment behaves more as you describe, or as it has in my experience. It's also possible that if your experience is dated, that things are now different.