how long and how much does it take bus ride from the city centre to the hong kong stadium?

Well, what do you mean the city center?
Take Tsim Sha Tsui Station as an example, no direct bus can take you to the stadium. You need to transfer.
Take Tsuen Wan Line to Admiralty Station and Island Line to Causeway Bay Station, Exit E. Then transfer to bus no.11, get off at Perkins Road, Tai Hang Road. After arriving, walk northwest around 330 yards (300 meters) to find the destination. The whole trip takes around 55 mins. The ticket fare is 15.4 HKD per person.