Getting a transmission flush every 2 years or 30,000 miles is standard. There are instances when a transmission may need flushed sooner. Checking the transmission fluid for the proper level and color (bright red) may help determine if it needs flushed.
When there is a build up of dirt, grease and sludge the transmission can show symptoms similar to low fluids. This can result in the transmission grinding or making strange sounds. If the color of the transmission fluid is not bright red but brown or black, the transmission may need flushed.
Other symptoms resulting from too much dirt or sludge may include problems shifting gears, slipping gears, surging of the vehicle or delay in the movement of the vehicle after being put into gear.