There is no one-size-fits-all answer for this.
You can easily determine the size of the OS as it is now using the
du commands. The real question however is how much more space are you going to use in the future for the OS ?
The size of the OS grows by packages you install and, to a lesser extent, by updates of already installed packages. In the 'Software Updater' or 'Software Centre' you can set up the update behaviour (automatic or manual).
So it all depends on what you plan to install additionally.
To create some more space for the OS you can remove some files:
- Downloaded packages are kept in
/var/cache/apt/archives and can be removed using
sudo apt-get clean.
- Kernel updates end up in
/boot and older kernels must be removed manually (they are never removed automatically).
- If space becomes a problem you can always move some directories of the OS from the SSD to your HDD and then softlink them back (
If you move your
home drive from the SSD to the HDD, then there will be more space left for the OS. The
home drive is often the biggest hogger of disk space. This might solve your problem already, at least partially.
My OS is also on an SSD (160GB) and occupies 140GB. My home drive is on a separate 240GB HDD and occupies 60GB.