Yes that is true 6 months is the limit, attached is the ADA guide book that also has some valuable information and will answer a lot of questions
As per ada 2017 guide book
"After your application has been processed, you will be eligible to test for a six-month period. If you do not schedule a testing appointment and take the DAT during this period, you will have to submit a new application and fee in order to take the test. The six-month eligibility period will not be extended, and you will forfeit your testing fee if you do not test within the six-month period."