Google releases its Android update, in this case, Android 6.0 Marshmallow. That is why the update weighs in around 1GB (according to personal experience on Nexus 6), though the size may vary from device to device. Already it has been suggested that this premium smartphone from Samsung will be in the stores earlier than previous iterations of the Galaxy S range. With regard when Android 6.0 Marshmallow would be available would depend on when the Samsung Galaxy S6 gets it.
The first thing you can count on from Samsung and its Galaxy Android 6.0 Marshmallow release is a roll out that spans across several months, maybe even an entire year. It seems an extremely long time ago that Samsung was bullishly proclaiming that it would overtake Apple at the head of the mobile marketplace. Of course, with an Android 5.1.1 update rolling out right now, it could be several weeks or months until we see some movement.