Most people would like to know beforehand how long it approximately takes to compile and install each package. But "Linux from Scratch" is built on so many different systems, it is not possible to give actual times that are anywhere near accurate: the biggest package (Glibc) won't take more than twenty minutes on the fastest systems, but will take something like three days on the slowest -- no kidding. So instead of giving actual times, we've come up with the idea of using the Static Binutils Unit (abbreviated to SBU).
It works like this: the first package you compile in this book is the statically linked Binutils in Chapter 5, and the time it takes to compile this package is what we call the "Static Binutils Unit" or "SBU". All other compile times will be expressed relative to this time.
For example, the time it takes to build the static version of GCC is 3.9 SBU. This means that if on your system it took 10 minutes to compile and install the static Binutils, then you know it will take nearly 40 minutes to build the static GCC. Fortunately, most build times are much shorter than the one of Binutils.
Note that SBUs don't work well for SMP-based machines. But if you're so lucky as to have multiple processors, chances are that your system is so fast that you don't mind.