While Microsoft has already announced that Windows 10 will be launched on July 29th, with Windows 7 and 8 users already notified to reserve their free copies of the new OS. We have not heard any official news as to when the Mobile version of Windows 10 will be released.
Windows 10 Mobile will be available late next quarter according to a presentation, a Microsoft employee made to one of their partners recently.
This means that we will most likely see a Windows 10 Mobile launch in September, we could also see the new flagship Lumia devices announced in September too, with a general availability in October?
Either way, Microsoft has 3 months to get Windows 10 Mobile ready.