The free upgrade lasts for 1 year after that you have to buy it
As linked in this Forbes article: http://www.forbes.com/sites/gordonkelly/2015/06/17/windows-10-free-for-1-year-what-happens-next/This is more than a one-time upgrade: once a Windows device is upgraded to Windows 10, we will continue to keep it current for the supported lifetime of the device – at no cost.
Once you upgrade within the first year for free, its yours forever at no additional cost.
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Windows 10 remains a perpetual license, which means, you can upgrade to Windows 10 and use it forever without it expiring or going into any reduced functional mode.