Normally 1 month, split half and half between the tenant and landlord, although this can be negotiated (ie: tenant can pay less if he negotiates with the agent without telling the landlord and the agent will receive less than 1 month of rent).
Keep in mind that in a vacuum, the agent's commission structure does not encourage them to bargain down the monthly rent because: 1) lower the rent, the less the agent receives in commission; 2) some landlords have multiple properties that are covered by one agent and the agent doesn't want to piss off the landlord; 3) the market supply of renters in HK is massive in some areas and so agents may rather focus on looking to steer other prospective renters into competing with you rather than help you out; and 4) agents sometimes just don't give a shit because they want to minimize the time spent closing a deal and maximize turnover and therefore only focus on closing deals rather than negotiating.
To get the best deals (I am not implying the cheapest apartment but rather one that I would consider to be good value based on my personal criteria), I have seen two strategies work best: 1) rent an atypical apartment (ie: for 3 bedroom apartments rather than 2 bedroom apartments) that renters typically don't rent so that you have less competition; and 2) utilize multiple agents for the same properties. I have had success with both methods (below market rates by 10% to 15%, below asking price by 20%).