My root particion (where I have the OS kernel, desktop environment and all SW installed) has currently 19GB (I have lot of SW, various compilers, development libraries, all major versions of gcc, etc..). It's of course fully updated to latest versions of everything.
But that's without the Android development stuff, that's in my /home, and that alone takes about 30-40+GB... because why f*cking not, dear Google... argh. It's driving me insane, how people carelessly fill gigabytes of storage space, most often in completely ridiculous and easily avoidable way.
So, anyway, if you are not developer, full desktop linux installation with KDE5 and being up-to-date would eat probably about 8-12GB at most.
JFYI ...