That depends on the route Nintendo goes with the Switch. If they follow the trend of constantly evolving mobile tech, I could see them doing incremental upgrades while adding new features to new models to keep the concept fresh. They would do something like the NEW 3DS where some games would be the higher end version only, while most would still work on both. Then when the Switch 3 comes out, 2 becomes the new low end and the current Switch is phased out. If they did something like that, I would say as soon as 2020 we could see a "successor" to the Switch.
Now if the follow conventional console cycles, I would say 2022 is a safe bet. Nintendo almost always follows a 5 year cycle and at this time the PS5 and NextBox would be entering full swing, and Nintendo will want to close that power gap for 3rd parties at that point. The current Switch could still be supported through cross gen releases of less demanding games for a few more years though.
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