how long does it take to go from tsueng Kwan o to butterfly beach at tuen mun

Well, there is no direct subways that can take you to the destination. You also need to transfer the public bus.
The whole time takes around 110 mins on the way.
About the transportation, take MTR Tsueng Kwan O Line to Tiu Keng Leng Station and take Kwun Tong Line to Prince Edward Station. Then transfer to Tsuen Wan Line to Mei Foo Station and West Rail Line to Tuen Mun Station, Exit C2. After that, take bus no.K52, get off at Butterfly Beach Park directly.
The ticket fare is 22.1 HKD per person.