the best answer you can get go to IEA singapore website. You will have a better picture. There no standard practice. Good agent follow the IEA guideline.

1 month for landlord for lease of first 12 month or up to 24 months and additional 1/2 month rental for every subsequent 12 months'lease or less. For tenant where rental is $2500 or less 1/2 month rental for less than 12 months' lease and 1 month rental for first 12 months or up to 24 months. This is the recommendation of IEA institute of estate agent.You can refer to www.iea.gov.sg

For rents up to $2500, the commission is half a month (1/2)of the rental amount for every 12 months of lease, from both landlord and tenant.