That's it, that's all this post is going to be, because I tried to Google this and couldn't easily find the answer.
Create a detail attribute using a wrangle with this code,
i@toggle = (@Frame/24)%2;
Which creates an integer attribute called toggle that cycles between 0 and 1 every 24 frames. Change the 24 there to duration you like and you can also increase the %2 to have it count up higher over that duration.
Then in the Select Input box on the switch put this,
Which reads that toggle attribute from that detail wrangle, which I called ToggleWrangle.