Across PlayStation's lifetime, the median gap between console generations is almost bob-on 6.5 years:
PS1 - December 1994
PS2 - March 2000 (5y3m)
PS3 - November 2006 (6y8m)
PS4 - November 2013 (7y0m)
PS5 - November 2020 (7y0m)
With almost exactly seven years between the last two generations it's reasonable to conclude a 6.5/7-year gap between the PS5 and PS6, placing it at May to November 2027 - and the latter more likely.
Each console was announced well before it arrived, although it depends on whether you count announcing its existence, name, or design as the announcement.
Sony first confirmed "PlayStation" in October 1993 (about 1y2m before it launched), revealing it in May 1994 (7m before launch). PS2 was first revealed in March 1999 (about 1y before launch), and PS3 announced in September 2004 (2y2m before launch) before a reveal in May 2005 (1y6m before launch). PS4 had very little lead time, announced in February 2013 (9m before launch), while the PlayStation 5 was first announced in April 2019 (1y7m before launch).
If November 2027 was to be the launch date (and most of the industry seemed to think late-2027 was about right, coinciding with the next XBox), that puts us right in the window for at least an announcement confirming its existence/name any time around now, and a design sometime around April-May this year.
That all, of course, looks like it's no longer the case.