How long does compiling minix take?
I believe it should take this long to do the job. It took me about 2 hours for the sh build.sh -mi386 -O ../build tools c to finish and it completed successfully and I got the build tools. After that process I ran the sh build.sh -mi386 -O ../build -U distribution command and that also took a number of hours. I ended up with the built distribution of MINIX which is what was expected of the process. It seems after that there is no way to turn this into an ISO though.
I don't know if the build script unsets MAKEFLAGS but you can do:
MAKEFLAGS=-j\ $(nproc) sh build.sh -mi386 -O ../build toolsnproc returns the number of cores in your CPU so it uses all cores and is faster.