How long can one have a colon obstruction from cancer (causing pencil thin stool) before it becomes fully obstructed (years, months, weeks?)estimate?

Likely weeks?: It depends on too many variables--pt's nutrition, weight, body size, other conditions, speed/aggressivity of the tumor, overall tumor burden, etc. However, this is an indicator of at least locally advanced process and need for oncology management with likely surgical intervention with end-of-life planning just in case things do not go as well as desired.

Days to weeks: It depends on how constricted the bowel lumen is and for how long you have had the symptoms of pencil thin stools. It is always a good idea to speak to your gastroenterolgist about this.

Not answerable : There is no good answer to this question. There are numerous cancers with different growth patterns. The location in the colon also affects this as a more distal cancer will obstruct sooner.