SBUs are Static Bash Units and they are our way of identifying how long a package takes to compile. Why don't we use normal times like anybody else?
The biggest problem is that times cannot be accurate, not even a little bit. So many people install LFS on so many different systems, the times it takes to compile something varies too much. One package may take 20 minutes on one system, but that same package may take 3 days on another (this is not an exaggeration). So instead we've come up with a Static Bash Unit or SBU.
It works like this: the very first package you compile in this book is Bash in Chapter 5 and it'll be statically linked. The time it takes to compile this package will be the basis and called the SBU. All other compile times are relative to the time it takes to install Bash. For example, GCC-3.2 takes about 9.5 SBUs and it's proven that this number is fairly consistent among a lot of different systems. So multiply 9.5 by the number of seconds it takes for Bash to install (the SBU value) and you get a close approximation of how long GCC will take on your system.
Note: We've seen that SBUs don't work well on SMP based machines. So all bets are off if you're lucky enough to have an SMP setup.