Again, I would completely prefer that we do not get midgen upgrades in 3-4 years. The generation should not be dramatically shorter than what we've been used to though (7 years, even 8 years from X360 to XBone) though, so like 4-5 years. But 6 years is reasonable for both Microsoft and Sony to start planning for them soon, like in 2021. The PS5 was started in 2015. Before we even heard about PS4 Pro. By the time of the first Wired article in April 2019, PS5 had been in development for roughly 3 and a half years. Sony and Microsoft and do post-mortems with various 1st party teams in 2021.
By mid 2022, Sony will be asking 3rd party devs what they want most out of a future console (more RAM, TBs of memory bandwidth, bigger GPU, etc) while starting the PS6 R&D cycle. By early 2023 Sony should have internal specifications and target a year. (i.e. Holiday 2026) and with AMD, begin design of the semi custom silicon based on AMD's CPU/ GPU/ process node / RAM configurations, etc roadmap. Chiplets, Infinity Fabric (4.0?), 3nm GAAFET, EUV, ultra high bandwidth HBM (maybe GDDR6X and HBM3+), and PCIE6, ETC and 20-30 Gbps SSDs will all be key and so will radically more capable multi bounce ray tracing that's also fast because of the comprehensive hw RT silicon in future versions of RDNA (RDNA 4 or 5). Dev kits could be in Sony 1st party hands by early 2025. Then 3rd party devs in mid-late 2025.
Announce in PS6 in early 2026. Cross-gen peroid will last 1 year with Sony 1st party studios, with their last PS5 projects releasing in 2027, like they did this year in 2020, except it'll be 6-9 months into the new generation. 3rd party games will be cross gen through 2027. By early 2028 the PS5/XSX (and especially Lockhart) are getting long in the tooth. Everything coming out in 2028 is PS6/Xbox NeXXXT. Come on we have to have it.