On January 21st MS will give us more details on Windows 10 for Phone and more information about consumer preview for PC's...So MS might surprise us by releasing the windows 10 preview on the same day(i.e., Jan 21st) or in coming weeks for both PC's as well as Phone's ..(take it with a grain of salt).Official Windows 10 may take several months for the wider roll out to existing lumias may be in Q2-Q3..
Hope Windows 10 preview will be released soon in the coming weeks..