While the constant prompts to upgrade to Windows 10 have been a little grating, you may regret ignoring them if you don't upgrade by July 29. After that point you'll have to pay for the privilege of upgrading to Windows 10.
Microsoft always intended for people to be able to upgrade for free for a year. A Windows 10 license will cost you the bones of €110 after July 29.
This deadline comes around a year after the OS was released and shortly before the anniversary update. This update will add new Cortana commands, a merged version of the Windows/Xbox One app stores, and advanced gestures.
If you haven't upgraded to Windows 10, you have until July 29 to do so without being charged.