We can establish a pretty good estimate for the season 8 release date based on its filming start date and how this syncs up with previous ones.
Update 30 August: THR is reporting, citing HBO sources, that production on the final season may end up being very long, running from October to as late as August 2018.
Production on season 7 began at the end of August 2016, with the show arriving 11 months later in July 2017.
11 months was a long wait for the latest season, but it might not be so bad this time around.