I think gearboax/transmission oil changes need not be so
frequent, but, it's usually 10K for most motorists, and for those
who have a particularly high standard of car maintenance, I'd say
change it every 7k. I change mine every 7-10K.
Along with all the others like spark-plugs, and oil filter. Rotate
and swap wheels, check and tighten handbrake lever, align steering
wheel, check undercarriage and tighten any moving joints.