I can't tell you about each month but I bought my mother a Z3 Compact around 14 months ago. It got Marshmallow ~6 weeks after its official release, and will have Nougat pretty soon now (so I heard on the net).
Even though I dislike Sony recently, they were awesome -- up to their Z3 phones and Z4 tablet releases. And they don't abandon their devices as quickly as everybody else.
Even though I dislike Sony recently, they were awesome -- up to their Z3 phones and Z4 tablet releases. And they don't abandon their devices as quickly as everybody else.