Double blind test- over a month- could this be a reliable test for any equipment?

Well, no, that's the point. Its not reliable. In fact its been proven not to work. This after all was the basis for some of the early codecs that were claimed to be scientifically tested identical sounding before everyone who actually heard them said hey no its crap.

At the very least, if you can't stand to be doing it for long that alone ought to be enough to know there's something wrong. 

@millercarbon

Just no. Have you ever intensely focused for 10 minutes straight? Because that’s what you are doing with double-blind listening, you are trying your best to be hyper aware of any sonic differences, hence why you need to take breaks.

There is nothing flawed with this method. That’s a cop-out for people who want it to be true that their $2000 speaker wire actually made a difference.  
 
The test needs to be double-blind, quick-switching (~10sec or less), and level matched. Now, being double-blind and quick-switching pretty much means the test needs to be automated.

In my experience, sometimes its possible to hear ’big and obvious’ differences right away, but on the other hand, sometimes it took me much longer time to hear and actually understand how some piece of gear affects the overall sound of my system.

Even more, if the ’differences’ were not so big in the beginning,longer listening helps me to determine if something sounds 'right' or not.

So, the blind test could help, but (imho) I would not consider it as essential for choosing some piece of equipment

This has been discussed ad nauseam. There are many perfectly good reasons why any test is unreliable, including double blind tests. In addition, there are many perfectly good reasons why testing over periods of time is also unreliable. For one thing test results oft depend on whose ox is being gored, but also the system used for the test, external factors, and a whole bunch of other things. And the sound changes all by itself day to day, hour to hour, month to month. So, the question naturally arises, when you check on the sound a week later or whatever, what are you actually hearing?

Yeah, made the scene
Week to week
Day to day
Hour to hour
The gate is straight
Deep and wide
Break on through to the other side
Break on through to the other side

And the sound changes all by itself day to day, hour to hour, month to month.

But if conducting the test over a month changes should average out. I would say if you cannot identify a difference in that scenario it is not worth the upgrade.

I think ideally it you could create a computer controlled AB that would only change (or not, double blind) after making a choice. Review the results and have an additional data point to make your decision.  You can always revert to feels if you like.