The official Twitter account for Sony UK has shared a date for when Marshmallow is expected to be released for the Xperia Z5, Z4 Tablet and Z3+. Should the information be accurate, owners of the aforementioned mobile devices can look to see Android 6.0 arrive on March 7.
@FalseNugget it's set to roll out from March 7th for Xperia Z5 series, Z4 Tablet and Z3+ – we will share confirmed timings when its ready— Sony Xperia GB (@sonyxperiagb) February 15, 2016
It's worth noting that this comes from the official Sony UK Twitter account and the exact date, as well as international availability, may be subject to change. That said, it's at least an indication that the company does indeed plan to release the update sooner rather than later. We'll keep you all posted should we learn more.