Windows 8.1's general availability was on October 17, 2013. Windows Phone 8.1's general availability was on July 15th, 2014. That is almost 9 months apart. If we assume that Windows 10 and Windows 10 Mobile will follow the same path, we can guess that Windows 10 Mobile's general availability will be around 9 months after Windows 10's which was July 29th, 2015. From this we can guess that Windows 10 Mobile will have it's general availability around the end of April 2016.