I have a contrary perspective.
It depends on your tolerance for flakiness and crashes. On my primary computers I run Linux, and I have zero tolerance. My machines are typically up for upwards of a year before I reboot it or there is a power failure. There have been cases where it has been several (years). Then something odd happens for the second or third time, I shift to another platform and start diagnosing. It is almost always the RAM (Was an Ethernet board once, and a switch once, which only needed new capacitors).
Given this perspective, I have a different take on number of passes. When I buy a new machine (literally or new to me on ebay) I run it for 1-3 or 4 weeks depending on my patience. After that, it has always run indefinitely. The only bad machines I have taken on were two Apple XServes from 2009 with 24GB of memory that I got for free. Each had one bank of bad DIMMs, and after removal they ran for many weeks before I got around to turning them off. With 8 physical XEON cores running concurrently that was quite a few iterations. It took a week or so to fail memtest86. Then I repeated the failure (Another many days! What a pain...), then I replaced the bad DIMMs.
Why do I do that? on 3 occasions I have had a machine become "unreliable" in that it would do something flakey or hang after a few weeks to a few months.
With a Linux machine, on my first such long run it found an error after 3 weeks, I replaced the DIMM, and it never crashed again. Similarly a 2 week incident.
I had a Toshiba laptop running Windows that would run for various times, hours to a couple of weeks. I found that it was lacking thermal compound and instead using some foam thing between the CPU and GPU and heat pipe. I replaced it with thermal grease (albeit not mechanically tight since the mechanical design assumed a thermal pad with non-zero thickness) and that helped considerably, but it still crashed on occasion. I threw it out.
Yes, if something is very grossly wrong, it will be found on the first pass, but usually it takes a few dozen hours. The machines I am talking about all take about an hour, hour and a half to run a single pass so that would be a few dozen passes.
I had some motherboards which all consistently failed memtest86 after a few months of running, yet memtest86 did not find anything (I don't recall how much patience I had with run-time). I dropped from 4 to 3 banks of memory and they never crashed again. My ASUS motherboards with the same exact chipset always worked fine with 4 banks. Both used Crucial memory.
I used to use the clock control feature of the BIOS to UNDER clock a step under the theory that this would make the machine more reliable if DRAM timing is the problem, but when I did have a problem it never helped, and the incident above is the only time I found that reducing the load on the common lines helped.
With Windows machines, I find that often they still pass the memtest86 for a couple of weeks test, but the machine remains unreliable under Windows. Sometimes an imperfect machine is suddenly reliable after a Windows release boundary. I had an issue before and during Covid with resolved on the fourth semi-annual Windows release - three releases with the problem! The same machine was suddenly rock-solid.
So if you reboot every day, or are happy saying "Oh, gotta reboot", and are not paranoid that some day the bad bit is going to be in your data instead of the instructions, then I would say to run memtest86 for at least a full day. In my experience most things are found in more than a few passes and less than a day. The information about the first pass being less thorough makes sense - I think it has always made it through the first pass or two. But a full day is not by any means conclusive. I am confident that a full month is, and I often compromise and run it for 2-3 weeks because I am impatient.
Finally, sellers of used computers typically swap things around, or even strip them into bins of parts then reassemble them based on what the customer wants, sometimes will little regard to static. I was told by one of them that the static issue was resolved sometimes in the 2000's and is not an issue any more. The truth is that static may wipe out a part, But most of the time it is just hot enough to mildly degrade a transistor only to manifest itself down the road. If you get a machine that has been running for a few years and nobody has taken it apart, chances are good that it will run roughly forever. Weak transistors are caused by impurities in the silicon crystal lattice, and the electric fields drive them to drift towards where they do the most harm. At higher temperatures, they drift faster. When there is a high current discharge (aka spark) they as well as the dopants that make the transistor a transistor are quite free to move around change the doping profile (Slope of the cliff). Picture a box of neapolitan ice cream with nice crisp boundaries between the vanilla and chocolate, now insert a little ni-chrome coil an inch in right on the boundary and heat it up to red-hot for a few seconds. What is going to happen? I had a laptop that I bought from a local recycler that would fail on memtest86 or crash every few days. I took it back and when I express concern about static (Looking at his process) he handed me another saying "Here, I have not touched this. It belonged to the IT manager of , that's where all of these came from and he gave me his last." That was in 2013. It is still running (2021), has not crashed yet. Almost worthless by today's standards, but it serves it's current purpose.
Hope this is helpful! Not welcome news, but this has been my reality. For reference, this is all based on something more than the 20 machines that I can quickly count.