Win 10 will run forever, Unactivated. Unlike previous versions, the small restrictions show from day one.
The watermark is randomly appearing. It's not there all the time.
Win 8.1 and earlier ran for 30 days with no restrictions. Then, increasing levels of pain, until it ultimately would go to a black screen every 2 hours.
Win 10 runs exactly the same as Day 1, seemingly forever. I have a 2+ year old Win 10 Pro, installed in a VM to test exactly this.
Installed on Dec 8 2016, running just fine.
Ultimately, though...the operating system is just as much a part of the PC as is the RAM, CPU, GPU, all them fancy fans...
It needs to be part of the overall budget.