Over time they grow just below 20 and can be maintained around 17. Anything more than that and something is seriously wrong.
Over time, the C:\Windows\Installer and C:\Windows\WINSXS folders will grow and grow as new patches are deployed and the old ones left in situ. MS is terrible about leaving the detritus of superseded and orphaned patches in place. It is not uncommon for these folders to reach 12GB and 20GB respectively*. That's 32GB of patches just in these folders alone, of which less than half can be safely removed - assuming that you have the knowledge and the will to do anything about it.
And that's before all the other parts of Windows itself, such as system restore points, that consume space.
OK, I'm talking about Windows 7 / 8 or Server 2008 / 2012, and it's going to take 5 or 6 years before it grows to that size, but Windows 10 is promised to be "longer lived", and that means more patches - which you will NOT be able to refuse - over a longer period.
As I've said, budget for 40-50GB for the Windows folder over a 5 year period.
Alternatively, you can do a completely fresh install of Windows and all your applications every few years, but with disk space so cheap, you would be mad to do that as opposed to just shelling out the cost of 30GB or so.
SD
* I have one Win 7 PC where these folders are 10 and 17GB respectively, after 6 years. Another PC that I have just rebuilt with a clean install of Win 7 has already reached 4.6 and 12.7 GB respectively.