What is the threshold where you would have to check baggage instead of going carryon only? Excluding special trip specific items that are too large or not allowed to be carried on.

By "threshold", consider length of trip, weather at destination, etc..

I have done a two week warm weather trip with just a carryon, but added a (small) checked bag for a two week cooler weather trip. In both cases, I did wash some clothes in the sink / bathtub.