How much of a 250 HDD is taken up with the op sys win 10 ?

Plain OS and nothing more about 50 Gb. With all applications, and to allow for big updates maybe 100 Gb and then your Videos, photos, music and docs on top of that.  250 Gb is fine for most users.

Windows 10 on a 35 Gb drive will not allow the semi-annual version upgrades so don't try that.

Even with 50 GB, you need plus 20 - 30 Gb for the semi-annual version upgrade. Windows.old is commonly 25 Gb when done.

I recommend getting adequate capacity as drives are cheap.

@john Nobody was suggesting he try that John, I was giving him examples of the different storage space windows 10 uses on different devices, rather than plucking figures out of thin air that have no basis in fact.
I regularly install windows 10 updates on devices that only have a 35gb SSD, but in no way was I suggesting that was the way to go.
How can Windows.old be 25gb on a 35 gb device with an OS installed?

Every one of my own and all client systems have Windows.old of between 15 and 25 Gb depending on what was installed.

I commonly see people here running of out space trying to update Windows 10 on tiny drives. So that is how I arrived at my recommendation above.

I did not pluck figures out of thin air.

I would say you're likely to be fine assuming you don't abuse it.  If you like to store pictures and videos on your computer or sync your iPhone, then you could have problems.  If you use it for work purposes and not (primarily) pleasure, then 250 GB should be more than enough.  Especially if you make a few minor adjustments.  

Personally, I use a Lenovo Helix and Surface Pro 4 with 128 GB of internal storage.  On the surface, I added a 128 GB SSD and redirected my Music, Downloads, Documents, Pictures, Videos, and other data to it.  This leaves the storage on the device free for the OS and program files.  And while neither of these systems has the full suite of what I MIGHT want to do (like my larger laptop with a 512 GB SSD), Both are very productive for me and have PLENTY of space - again with 128 GB drives!

and of course, you can Always use an external drive for stooring extra data
or install a bigger SSD later, or even a 2nd one - depending on the model

Also, with regards to the Windows.old folder, that can be removed so long as you have enough space to upgrade.

Here's the specs (space you'll need) to consider for being "able" to upgrade - as in how much space you'll need for upgrades to be able to install.

https://www.microsoft.com/en-au/windows/windows-10-specifications

Cheers...