Most likely when PS5 and the next Xbox will release. They will probably try to act like Switch is now a 9th gen console and can compete with the other 9th gen consoles with their new model.
Switch is not directly competing with current gen stationary consoles, but rather it is complimenting them like other portable handheld systems did in the past. If/when Nintendo does make a revision or successor to the Switch, it will be in order to prolong platform sales and potentially expand the system's b ase as opposed to try to compete with systems that in reality will not be in the same device catagory and hence not in direct competition to the Switch. That would be like Apple releasing new iPhones to compete with new desktop computers.