Weeks recommends having your transmission serviced every 25,000 miles. Transmission fluid has a viscosity, just like oil does and breaks down over time.
Transmission fluid should be changed periodically. Your owner's manual should give you the recommended intervals which could be anywhere from 15,000 miles to 100,000 miles. Most transmission experts recommend changing the fluid and filter every 25,000 miles.
Few transmissions have drain plugs to drain the old fluid. In order to get the fluid out, the technician removes the transmission oil pan. This is quite a messy job and generally not recommended for the casual do-it-yourselfer. Even if the transmission has a drain plug, the only way to also change the transmission filter is to remove the pan. When the pan is down, the technician can check for metal shavings and other debris which are indicators of impending transmission problems.
In most cases during these transmission services, only about half the oil is able to be removed from the unit. This is because much of the oil is in the torque converter and cooler lines and cannot be drained without major disassembly. The fluid change intervals are based on the fact that some old fluid remains in the system.
When the transmission is serviced, make sure that the correct fluid is used to re-fill it. Each transmission manufacturer has their own recommendation for the proper fluid to use and the internal components are designed for that specific formula. GM usually uses Dexron, Ford uses Type F, Toyota sometimes uses Type T. A transmission will not work properly or may even slip or shudder with the incorrect fluid, so make sure. Your owner's manual will tell you which fluid is required.