Thanks. Interesting answers, and it obviously varies a lot.
The reason I asked was because I ate my "emergency" snack (cereal bar) yesterday 1 hr 10 mins into my ride in what was very much NOT an emergency. I just felt like I needed it.
I'd tried a new route, got a bit lost, found myself plodding up a longish, steepish hill. When I got to the top, I figured the ride was going to go over 2 hours, I was feeling a bit sorry for myself because it wasn't going to plan, so decided to stop and eat. It was only a 100 cal cereal bar, but it did seem to pick me up a bit (probably all in the mind!) and I ended up getting home in 1 hr 50. It would have been interesting to see how long it would have taken me if I hadn't eaten it.
I generally carry something with me because I figure if the dreaded p* strikes, by the time I've had a fight with the tyre getting it off the wheel and back on again, which usually involves a lot of trapped fingers and swearing, I'll be ready for something to eat! I don't generally plan to eat unless I'm going to be on the bike over 2 hours.