How long is it supposed to it take for a child to be tested for autism?

Screening Tests. There are screening questionaires, that can be administered in your pediatricians office, such as the mini- chat, that takes a few minutes. It has been tested and validated to provide reasonable accuracy if checked at 18 and 24 months of age.
Depends. Diagnosing autism, especially with significant symptoms present, takes very little time when an expert is invovled. What takes time is ruling out other problems and possible causes of the disorder and assessing the child's cognitive strengths and weaknesses to help parents plan next steps. As others have indicated, that can take many hours and multiple specialists.
Depends. In mild cases observation over time may be needed to determine a diagnosis. In severe cases it may be easily apparent. Testing per se, may take several hours depending on how many professionals are involved. Developmental pediatrician or neurologist, speech/language pathologist, audiologist, psychologist/psychiatrist, learning specialist. Each test could take many hours.