The more roots you can save the better it's chances of survival.

in other words, none.

I agree with islandbreeze and lzrddr. It can recover from getting it's roots cut off, but it will be a lot healthier and happier if you don't disturb the roots too much. I would repot it in a pot big enough to give the roots lots of new soil to grow into.

Good luck!
-Alex

1/3 is a safe amount.

Jane