You say "Marshmallow" in the title, then mention an update form 7.0 to 7.2.
Just a few clarifications so that others aren't confused. The S8+ (and the S8) came with Android 7.0 which is known as "Nougat", or just "N". "Marshmallow" was Android 6 (or "M").
There is no 7.2. Nougat was updated by Google to 7.1, 7.1.1 and 7.1.2, but Samsung has chosen to not update the OS for the S8 and S8+ beyond 7.0 at all (the Note 8 does get 7.1.1). So, if you meant "7.1.2"... it ain't coming.
Samsung has had a beta program for the new Android "O", or Oreo, which is 8, for several months now - but actual release date is unknown. The current best guess is that they are close to having a finished product and may have something by the end of this month or the next.
However, while Google has now updated Oreo to 8.1, it appears that Samsung will again stick to 8.0 and not go beyond that.
That covers Samsung.
There is still AT&T that always gets in the way.
Personal experience is that AT&T will say they are "testing" it and will hold it up for anywhere from 3 to 6 months. So, expect it sometime around August, but be thrilled if it comes out sooner, and hammer them if it's later.