IIRC, there is no time frame stated in the rules. As a practical matter, you will need to allow enough time for the change to be done and ticket reissued. This will very according to how the change is being made (TA or airline, which airline, availability of specialized/knowledgeable agents, like AA RTW desk, in person or by phone, going from a transit to stopover, etc.)
Over the years my experience for a simple date/time change has ranged from minutes to 3 days depending on the above factors.
Obviously changes to the first segment will require repricing and take much longer; changes due to irregular ops may be done on the spot at the airport.