Colon resection of descending colon. how much can be removed and still be reversed ?

What's gone is...: Gone, but can function quite well with what's left. Colon is about 5-6 feet long. Can do well with the remaining amount. Any surgery can cause internal scars (adhesions), which could cause obstructive issues in the future (or not).

Nearly any amount: As long as a functioning anus is remaining, the entire colon and rectum can be removed and intestinal continuity restored by a ileoanal anastomoses.