You end up with a permanent small water mark on the bottom right-hand side of the screen as a reminder to activate. Currently, you can keep it like this indefinitely.

There are tricks to make the watermark invisible without activating Windows. It works just fine if Windows 10 is not your primary operating system and you don't plan to personalize your operating system after fiddling around a bit just after you get it installed. You can personalize it for about a couple months before the activating logo shows up; at least that's how I always experienced it.