March 1st or March 8th, 2024, for three reasons.
1) The tech should have progressed enough to a point where we can get a handheld/portable system with PS4/Xbox One level hardware with an acceptable battery life.
2) The 3DS managed to get a 6 year life cycle and the Switch is on pace to far exceed the 3DS, so anything less than 6 full years for the Switch would be a severe disservice. So, it 2023 should be the earliest it launches, but I feel the Switch should naturally go longer than that with how well it is selling and I am confident it's legs will fair better than the 3DS' legs to justify the extra year.
3) It would be closer to the launches of the PS4/Next Gen Xbox than the Switch was to the launches of the PS4/Xbox One. Those systems launched in November 2013, the Switch launched in March 2017, that's 3 years and 4 months between them. If the Switch 2 were to launch in March 2024 (what I'm expecting), and the other two systems launch in November 2020 (What I also expect), that would also be 3 years and 4 months between them.
Hardware Comparison Threads:
Current Thread
Is Hardware Getting TOO Powerful?
Older Threads:
PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017