I don't know what the numbers are, but there are a lot of things you can do to prevent burnout. One is to make sure that you join a group that has other people doing a significant amount of critical care time so that you don't have to do too much of it per year. (too much of a good thing can make people burn out!). Also, if you join a group with an overnight intensivist moonlighting/call schedule, most of your nights can be free even when you're on service, which will make you much happier. Last is to make sure that you have a backup that you enjoy doing so that if you have a really bad 2-4 weeks in the ICU, you will be able to relax and enjoy yourself during your time in between being on-service, and will be ready to go for your next round of ICU.
Personally, I think every new field gets a lot of hype about "burnout" for some reason or another. Maybe because it's usually younger physicians who are in those fields since older ones don't want to switch, I don't know. But the issue of burnout has been raised for EM, hospitalist medicine, and critical care -- all of these fields in their current forms are less than about 20 years old. (critical care has changed alot in the last 5-10 years, so I'm counting it as a young field). As we have seen from EM physicians and now from hospitalists, the burnout concern that everyone was talking about when these fields first started up is not really an issue. I think that's because they both have almost a shiftwork mentality so that physicians don't have to take their work concerns home with them. Both fields are very stressful while you're there, but then there is a LOT of free time associated with them, allowing people to actually have lives and take lots of vacations -- I think this has been the big factor in preventing burnout.
I think critical care is evolving into a similar structure as EM or hospitalist medicine -- people are very concerned about the longevity of the physicians in the field, so they are doing everything they can to try to ensure that intensivists stay in it.
So I wouldn't worry too much about what other people not in the field are telling you -- you will be able to work it out, and you will see that as this field matures, there will be more and more intensivists in it for the long haul.