Gentoo’s installation procedure is one of the most complex we’ve come across, and it’s largely carried out manually with little use of helper scripts. Because it’s a source-based installation, you can end up waiting for, say, half an hour while certain packages are compiled and installed. If you choose the wrong option, this obviously adds to the time. We reckon that a user who understands manual partitioning and compiling the kernel would typically need a couple of nights to install the system. A less advanced user might need a week.
Arch Linux installation is also a largely manual affair, but it doesn’t involve compiling packages from source. We reckon it might be the best balance between having full manual control of what you are installing and learning how Linux works with building a usable system with a reasonable amount of effort.
Void Linux is similar. Where appropriate, standard tools are used, but some of the install is made easier with custom scripts. Getting out of things like manual keyboard setup is convenient without making us feel too separated from the experience.
If you’ve been with Linux since the early days, Slackware’s installation will be a trip down memory lane. Some manual work from the command line has to be done to install the bootloader Slackware’s installation utility does a lot of the work for you, which might be what you want, but it does detract from the full manual installation experience.