How long is the bus journey from Taiyuan's East Bus station to Wutai Mountain?

Hello everyone. I have been getting different answers varying from 3 to 5.5 hours ?

If I catch the latest bus 1830 and it takes 5.5 hours, I won't get there till 11:30 p.m. then ?

Also, is it necessary to buy the bus ticket in advance ? Can I buy it online ?

Many thanks

Dee