5 years seems to be the average, but remember that average means just that.
I worked 15 years in the field, but 10 years of it was with a special event service, if not for that, I would have burned out sooner. Posting to this group is the first thing EMS related I have done in over 10 years- I still have some interest, but I will never work on an ambulance again.
I don't think it was stress, it was just dealing with people.
I knew I did not want to stay in EMS any longer, but I did not want to do anything else in the medical field- I just got to the point I did not want to deal with people.
I finally decided on career change- I am just completeing 12 years in my new field, I have no intention of changing, and I actually lead a normal, happy life, something I never could manage on an ambulance
I think your best way of avoiding burnout is to have interests completely outside of EMS. Full time and volunteer is to much, fyrdog is right- you want to look forward to your shift.
Fyrdog summed it up nicely- I could have written his post myself.