There are several ways you could approach this:
SDL doesn't send 'mouse motion stopped' events but conceptually, a 'mouse motion stopped' event is a frame where you haven't received a mouse motion event. If you have a frame update loop, keep track of whether you've received a mouse motion event in the previous frame and update your menu timer accordingly.
Simply reset your menu's timer every time you receive a
SDL_MouseMotion
event. It's not elegant but it should work. Run the menu timer as soon as you receive the first motion event and just reset the timer each time you receive a subsequent one until it expires and you hide the menu.