The Wii was released in 2006, sold 100m units, and then the Wii U came out in 2012. The Switch was launched in 2017, is on pace to sell 100m+ units, and so 2023 seems like the logical option.
The two unknowns for the Switch successor are hardware costs and the possibility of a half-step Switch Pro that extends the life of the Switch. Regarding costs, Nintendo needs to get to the point where it can sell a hybrid for $300 that can run high caliber PS4/X1/PC games (e.g., RDR2, Death Stranding, Sekiro) with minimal compromise. It should also be able to run games like Cyberpunk 2077 with dedicated porting.
However, if they release a Switch Pro with more internal memory and an improved SoC in 2021, then the Switch 2 might not arrive until 2024 or 2025. Until 5G is widespread and xCloud/Stadia/PS Now perform spectacularly, Nintendo has a monopoly on handheld gaming. Even when those streaming services get amazing, Nintendo will still be the only company with bespoke handheld hardware which means the Switch will sell well for a long time.