If your crohns has been confine to your large intestine (aka crohns colitis) then there's a very good chance it won't return (which is why some docs will actually do j pouches for those who's crohns has been confined in this way for 5 years or longer). If it does manifest itself along other areas of your digestive tract, there's really no guarantees as to how long before it returns unfortunately :(
And you're right, since UC only actively attacks the large intestines, once they're gone, the UC has no place to attack (altho we can still get the other stuff, ie arthritis, joint pains, etc)
I have had an ileostomy for 31 years now due to UC.
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