Several answers to this, mostly with the proviso "it depends".
Firstly, if you have dissected and are NOT diagnosed with an AD (and it is a difficult diagnosis), life expectancy falls rapidly - roughly 1% per hour mortality. i.e. in 2 days half of sufferers will have succumbed. If found and treated I think the stat is something like 70% survival at 1 year.
Also depends on whether it is type A or type B, age, blood pressure, cofactors such as aortic valve problems, whether the tear affects side blood vessels to legs, kidneys, brain, eyes, heart etc.
If you are repaired well by an expert surgeon, there is a good chance you will lead a life pretty much as long as it might have been anyway, and hopefully even with a similar activity profile.
Much better though if you are operated on before dissection, at the aneurysm stage.