Replacing a transmission can be a lengthy and complex process. Depending on the type of vehicle, it can take anywhere from 4 to 12 hours or more for an experienced technician to replace the transmission. First, the mechanic will need to remove the old transmission from your car and then inspect it for any signs of damage or wear. Once this has been done, they'll check all related components in order to determine if any repairs needs to be made before installing a new one. After that is complete, they'll proceed by carefully mounting and connecting up all necessary wiring and hoses while ensuring there are no leaks in engine oil or other fluids. Finally, once everything is securely installed, they will test drive your car in order to make sure the replacement was successful and that there are no issues with performance or shifting gears properly. All told, replacing a transmission usually takes at least half a day which is why it's important you find an experienced professional who knows exactly what they're doing when tackling such a project.