Automatic transmissions require very little maintenance and can last a long time, as much as a couple hundred thousand miles. But you do need to have the transmission fluids flushed. An initial transmission flush should typically take place around 60,000 miles, and usually every 30,000 miles after that.
Manual transmissions are operated by clutches, which can wear with use, and typically fail around 150,000 miles. There’s really not a lot of preventive maintenance on manual transmissions. However, if needed, we may drain and fill the oil, or we may also service the hydraulic fluid that operates the clutch pedal.