A Microsoft Support Engineer recently posted a list of Windows 10 builds and the dates that each will stop booting. According to the engineer, you will start getting warnings around two weeks before the license expires. If this happens, your computer will begin restarting automatically every three hours. Two weeks later, your computer will not boot and you will have to download the ISO from a different one, which is definitely something you want to avoid if you are using Windows 10 as your daily driver. According to the chart, build 9841, 9860, and 9879 will stop booting on the 30th of this month, which is right around the corner.
If you are currently running these builds and haven't updated (or if you are having problems updating) you can download the ISO for build 10041 which will keep you in the green for the next six months.