The frequency of checking and changing transmission fluid in a car depends on several factors such as the make and model of the vehicle, driving conditions, and the type of transmission. It's essential to consult your vehicle's owner's manual for specific recommendations, as they provide the most accurate information for your particular vehicle.

However, as a general guideline:

1. Check the fluid regularly: It is advisable to check the transmission fluid regularly, at least once every month, to ensure it is at the correct level. You can do this by locating the transmission dipstick (consult your owner's manual if you're unsure) and following the instructions on how to check the fluid level.

2. Change the fluid based on the manufacturer's recommendation: Most modern vehicles have transmission fluids that are designed to last for a long time. Manufacturers often recommend changing the transmission fluid at specific intervals, such as every 30,000 to 60,000 miles (48,000 to 96,000 kilometers), or every 2 to 4 years, whichever comes first. However, some manufacturers claim their transmission fluids are lifetime fluids and do not require regular changes. Again, consulting your owner's manual will provide the most accurate information for your vehicle.

3. Consider your driving conditions: If you engage in severe driving conditions, such as towing heavy loads, frequent stop-and-go traffic, or driving in extreme temperatures, you may need to change the transmission fluid more often. These conditions can cause the transmission to work harder and generate more heat, which can degrade the fluid more quickly.

Remember, proper maintenance is crucial for the longevity and performance of your vehicle. Following the manufacturer's recommendations and regularly checking the fluid level will help ensure your transmission operates smoothly.