Rode 4 miles today in a slow, steady rain. Put on my rain jacket and pants and stayed dry. Took things a little slower and had no problems. I did wipe down my scoot when I got back to my garage.
The longest rain ride I had was 400 miles on my Kawasaki, and it was not a bit of rain, it was pouring rain all the way. I had water resistant jacket, trousers and rain overalls on top of that. It was especially awful to ride because it was in the summer, so it was warm, rainy and just plan disgusting.
I was riding between 60 and 100 mph. After a while you get so used to rain so you can almost ride as if it was dry, especially on good roads. Not a single mile was done on highways, it was mountain/coastal scenery all the way.
Caveat: this would not be possible on touring scooter tires, at least I think so. Also, I was much younger then. Memories...
I have daily commute of 8 miles to office and 8 miles back and usually it starts raining when I exit the house, or office building...