Release to Manufacturers: March
Release to Developers: April
Release to Users: May
According to well-known MS insider Mary Jo Foley,
From what I’ve heard, Windows Phone 8.1 won’t be available to consumers until later this April, at the earliest, though developers will likely have access to the bits sooner than that.
Even our sources tipped us a Q2 2014 release of Windows Phone 8.1 update. So, a consumer release of Windows Phone 8.1 in the month of May looks probable. That’ll be truly awesome, as it is hardly 3 months wait then and with developer preview possibly in April, for enthusiasts it is just 2 months away.