About 1 hour into a 2 hour 30 minute flight I started feeling the need to go (just a no. 1, fortunately), got steadily worse over the next half hour to point I was seriously considering using my fuel tester! (I've got on of the larger ones with the gauze on the top)
It did seem to stop feeling the need so bad just after this, probably something to do with getting into complex airspace and being distracted by radio etc. I made it to my destination without needing to use my fuel tester, though it felt like the flow wasn't going to stop when I got into the toilet!
I'll definitely carry an empty bottle from now on!