Check owner’s manual for your car as your vehicle may have different intervals for maintenance.
Suggested every 60,000-100,000 miles due to more heat generated within an automatic transmission case. That heat can cause the fluid to break down and be less effective in lubrication.
Intervals for Manual Transmission Service:
Transmission fluid is easily contaminated by metal particles as the bearings and gears wear down. These particles float around and cause the fluid to not lubricate as well as it did when fresh.
Manual transmissions generally should have a service every 30,000-60,000 miles.