About 10Mb for a very minimal installation, suitable for trying Linux, and not much else.

You can fit a typical server installation, including the X Window Systemt GUI, into 80Mb. Installing Debian GNU/Linux takes 500Mb–1GB, including kernel source code, some space for user files, and spool areas.

Installing a commercial distribution that has a desktop GUI environment, commercial word processor, and front-office productivity suite, will claim 1–1.5 GB of disk space, approximately.