3h is enough, i made it in 3:30h.
Apply for permit 7 to 30 days ahead (good luck it is not easy to book, since the entrance is limited):
https://npm.cpami.gov.tw/en/
The entrance fee of NTD 200 you pay at sight. Don't forget your passport!
...
https://www.youtube.com/watch?v=kUjMYzb3xx4