The absolute answer to the question of how long hedgehogs can live is that
nobody is really certain.

The average lifespan for African pigmy hedgehogs in captivity runs from 4-6
years, with some living as long as 8 years.  From what I have read, this is
already far better than the average of 1-3 years in the wild (though this is
due more to predation, disease, and other environmental factors than to old
age).

African pigmy hedgehogs have not been kept in captivity for many years
as yet -- that along with the fact that we are still in the early stages
of learning how best to care for them, is likely to allow their longevity 
to continue to increase over the upcoming years.  

In addition to the good care aiding this, the fact that hedgies are starting
to settle down and become much less nervous will likely also help
considerably.

All in all, their lifespan is quite long compared to many smaller mammals,
which means that there's a good chance you can continue to get along with
your prickly little pal for many happy years.