There was about 15 months between the DS and DS lite and it wouldn't surprise me if the revised Switch comes out about 12 months from now.
I see it smaller than current Switch though without removable joycons but still dockable for the big screen output. It won't have the table top leg it will be more a dedicated handheld with superior battery life. I think it will be cheaper, and more popular in Japan. More like a 5.5" screen or possible smaller.
I think the current size of the Switch was forced on Nintendo by necessity but it is not the size they want to be it.