There is a mistake in the build system usage guide where the explanation for the typical build steps are not in the order of the steps.
For instance, the code revision steps are before the feeds, but in the explanation the feeds come before the code revision.
This mistake wasted about fifteen minutes of my life.
There are other mistakes too, but I am out of time. 
In the steps, the working directory is 'openwrt'. In the explanation its ''. The name of the working directory should be consistent in both the steps and the explanation.
Edit: A design problem in this forum software is preventing my post from displaying the way I wrote it, so the name of the working directory in the explanation that I quoted is not displaying.
I have just wasted six hours of my life over this line:
"After a successful build, the freshly built image(s) can be found below the newly created <buildroot>/bin directory."
It should be changed to:
After a successful build, the freshly built image(s) can be found in directories located in 'openwrt/bin/'.
Higher priority must be given to OpenWrt's documentation. Its pretty bad. People would get fired over this.
No, it really shouldn't. The line that is in the documentation is correct. It's for the end user (i.e. you) to be aware of their own directory structure and substitute their own directory when reading through the instructions.
Wrong, the step does not use that working directory, it uses 'openwrt/', therefore the path in the explanation is inconsistent with the step, so they should be one or other, together.
And given the step doesn't tell the user to choose their own path, 'openwrt/' is correct as the step is literal as it is instructional to the end user on what commands to use for the demonstration.
The example uses 'openwrt/', but it's just an example. The detailed instructions all refer to <buildroot> because it's open to the end user to change that if they choose to do so. If you don't then yes the directory will be openwrt, but it doesn't alter the fact that as the end user you should be substituting that into the instructions as you read them.
The instructions for the steps doesn't tell the user that it is open to them, the steps are directional for the purpose on demonstrating how to use the software, therefore, the reference to the directory in the explanation needs to be consistent with the directory in step.
"The build steps are explained in detail below."
Another problem with this build system usage guide is that the explanation covers things which are not in the steps. This overloads the person trying to understand the steps.
The example at the top of the guide is not instructions or steps, it's an example of the 'typical' process followed in a 'typical' build set up. That's it. For some use cases you might be able to copy them verbatim and it'll work fine, for others it won't. That's why there's detailed instructions/explanations provided.
"Build system usage
Do everything as an unprivileged user, not root, without sudo.
Make sure there are no spaces in the full path to the build directory.
Typical build steps in a set up build system:"
Sounds pretty instructional to me, and it says they are steps.
Anyway, the guide confused the hell out of me and is a terrible experience. 7 hours and counting... 8 compiles... no result.
I did this exact learning curve my self for about a month ago and I am definitely not an Linux expert and I screw Linux things up from time to time.
But I don’t see how you can waist 6h on this little setup detail without figuring it out?
You really need to be able to get information from different sources and build the puzzle on your own to build images or code in general.
How are you going to handle all the other developer guide pages for the build process because they have no build order at all and you will have to search and find them yourself when you realize you got stuck.
And after the first build in the build root you need to be able to jump freely in the build setup process.
You need to hire someone first to be able to fire them
It's fair to point out where the wiki can be improved. It is not fair at all to expect a community of volunteers to have a product without rough edges. I suggest applying for a wiki account and fixing the things you think are wrong.
But they do. It says right there:
git clone https://git.openwrt.org/openwrt/openwrt.git [<buildroot>]
which means "...optionally provide the desired location which we'll refer to as <buildroot>".
Build time mostly depends on your hardware.