I would give yourself 2 or 3 hours to have time to catch a planetarium show, and the live actor frontier homestead exhibit along with the awesome dinosaur exibits and the current history of the guitar exhibit. The planetarium is a very cool contrast to the other things and does not cost extra.
There is also a kids play/explore area upstairs that is really well done.
It depends if you like to stop and read every sign or just breeze by. When I go with my kids (age 2-10) we spend at least 2 hours and don't stop to read too many signs. There's 2 displays (dinos and historical MT) and one more rotating display that take w while to walk through, then a fabulous kids interactive play area upstairs. Oh, and the planetarium that has shows that run about a half hour each.
If you want to check out everything, an hour to an hour and half.
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