Most older cars and trucks,  require  a schedule maintenance for the replacing of the transmission fluid and its filter.

The newer cars require a longer schedule maintenance time for  replacing the transmission fluid.Please check with your owners manual and it will advise you accordingly.

My personal opinion is to change the transmission fluid and filter approximately 1/2 to 3/4  the time the manufacturer recommend in the owners manual.

Changing the transmission fluid, adds years of life to your transmissions.