It depends if you're just there to look at the cars and all you can probably do it in 45 minutes to an hour. But if you sit there and read about each and every one of the 68 to 70 cars about 2 hours. Keep in mind you're not allowed to touch the cars or open their doors you just view on them and read about them.
2 hrs
If you read most the signs about two hours
A couple hours is good for a leisurely browse. My partner and I spent that much time easily when we went.
About and hour
About an hour+
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