Windows does not have a "set" amount of RAM it uses. Windows optimizes ram usage based on your hardware. For instance, if you have 32GB of memory, Windows will use a lot more memory to improve performance, but if you have 4GB then it will use less because it's optimizing usage.
How much the actual OS uses will vary, depending on hardware and drivers installed, and what services are enabled.
As for whether it grows over time.. well, it changes over time, not sure it's always growing... but when new features are added to the OS, they will obviously use additional memory. However, the general trend is more over time, and that's simply because ram is so cheap, it makes much more financial sense to utilize more ram than to require more CPU to increase performance.
Also, the longer the OS runs without a reboot, the more memory it will consume in general as it fills more buffers and caches.. this memory is considered "on demand" and can be discarded if needed by applications, so there is no logical reason to NOT use any and all memory the system has, if it can be put to use to improve performance.