A statement is rationally possible when its content does not involve a contradiction as such. It is customarily possible when it also fits what may happen under ordinary circumstances and familiar causes.
A long journey is rationally possible. Attributing that journey to someone in an impossibly short ordinary span moves the claim toward ighrāq.
Customary limits depend on the stated circumstances and means. The rhetorical distinction concerns what the claim exceeds in its own context, not a universal numerical threshold detached from circumstances.