Visa for Belgium nationality

I should go to China departing from Taipei for a trade show.
I have a Belgian Passport and would like to apply for a business visa to
go to the tradeshow in Beijing for only 4 days.

I cannot apply in Taipei, only in Hong Kong, but how many days should I stay in hong kong to
get it? Is arriving one day in the morning, giving in application into the office and then the next working day
I have the visa? So that arriving on monday, getting on tuesday and flying out on Wednesday?