Transmission service will depend on a lot of factors, such as ambient temperature when driving, overall mileage and the kind of car you drive as manufacturers recommend changing the transmission oil every 50,000 to as high as 120,000 miles.
Whether you have a manual or automatic transmission in your car, problems are inevitable, as the car ages and should be given attention once discrepancies are observed. Such problems may include hesitation, lag in shifting or abnormal noises that stem from the transmission itself. Most problems could be prevented if the owner exercised a modicum of care in use and maintenance of the car. Whatever the occurrence, any problems that may persist or any unusual activity observed in the transmission should be consulted with the dealer or service center at the least possible time. Simple adjustments or a simple change in transmission oil will solve most problems, but an expert and honest mechanic will give you the whole picture upon inspection.
The transmission is a sealed system and a significant amount of fluid loss will indicate a leak. Pop open the hood and pull out the ATF dipstick to check for proper fluid level and condition. Add transmission oil when necessary, but remember not to add too much. Insert the dipstick all the way and pull it out again to see if the fluid is at the right level. The dipstick will have markings that will determine proper oil level.
The oil inside the transmission is usually bright red and would change color as you pile mileage in the car. A black or dark brown oil with a burnt smell should be consulted with a mechanic at once.
Periodic maintenance according to manufacturers recommendations should be followed to prolong the service life of any car. It is good practice to check the oil in your transmission every time you check the oil in the engine.