Time between the start of recording & album release (according to Wikipedia):
WD&DU: July 1988 - March 1989 (8 months)
I&W: October 1991 - July 1992 (9 months)
Awake: May 1994 - October 1994 (5 months)
FII: June 1997 - September 1997 (3 months; but the recording was delayed by the record label, so they had plenty of time to rehearse the material)
SFAM:
- October 1999 (
)
SDOIT: March 2001 - January 2002 (10 months; double album)
TOT: March 2003 - November 2003 (8 months)
Octavarium: November 2004 - June 2005 (7 months)
SC: September 2006 - June 2007 (9 months)
BC&SL: October 2008 - June 2009 (8 months)
ADTOE: January 2011 - September 2011 (8 months)
DT12: January 2013 - September 2013 (8 months)
TA: January 2015 - January 2016 (12 months; double album)
D/T: June 2018 - February 2019 (8 months)
With this in mind, if the band is entering the studio in September-November 2020 & writing all their material from there (which I believe has been their process for every album since SDOIT), then a May-July 2021 release date is most likely.
Edit: Also just for fun, these are the actual recording times, not counting the extra months for distribution & promotion.
WD&DU: July 1988 - August 1988 (1 month; written pre-studio)
I&W: October 1991 - December 1991 (2 months; written pre-studio)
Awake: May 1994 - July 1994 (2 months; written pre-studio)
FII: June 1997 - July 1997 (1 month; written pre-studio)
SFAM:
SDOIT: March 2001 - August 2001 (5 months; double album)
TOT: March 2003 - September 2003 (6 months; but written in only 3 weeks)
Octavarium: November 2004 - February 2005 (3 months)
SC: September 2006 - February 2007 (5 months)
BC&SL: October 2008 - March 2009 (5 months)
ADTOE: January 2011 - May 2011 (5 months)
DT12: January 2013 - May 2013 (5 months)
TA: January 2015 - September 2015 (8 months; double album)
D/T: June 2018 - September 2018 (3 months; but written in only 18 days)
Edit 2: & here's the time between ending of recording to album release
WD&DU: August 1988 - March 1989 (7 months)
I&W: December 1991 - July 1992 (7 months)
Awake: July 1994 - October 1994 (3 months)
FII: July 1997 - September 1997 (2 months)
SFAM:
- October 1999 (
)
SDOIT: August 2001 - January 2002 (5 months)
TOT: September 2003 - November 2003 (2 months)
Octavarium: February 2005 - June 2005 (4 months)
SC: February 2007 - June 2007 (4 months)
BC&SL: March 2009 - June 2009 (3 months)
ADTOE: May 2011 - September 2011 (4 months)
DT12: May 2013 - September 2013 (4 months)
TA: September 2015 - January 2016 (4 months)
D/T: September 2018 - February 2019 (5 months)