Closer, the better. No need to ride up and down the mountain more than necessary. Take some bottled water as drinking fountains are far apart.
You can schedule them as close as you'd like - however it would be reasonable to allow at least 20 minutes in between tours (for bathroom breaks and the possibility of a delayed bus).
Each tour is about an hour apart. If you stay at the top I would think you should space them out at least 30 minutes from the end of one to the beginningof the next one. If you want to come back to the visitors center add 15 minutes each way.
By bus 10 to 15 mins.
I highly recommend at least 30 to 45 minutes so you can look around on your own and take your time when taking pictures. For sure stay at Top. There's plenty to see while you wait.
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