You can find out the last day of validity for your visa by using the Visa Pass Validity tool.
For nationals of countries not requiring entry visas, a 14-day Social Visit Visa is typically issued upon arrival. One can extend this visit pass for a longer period by applying for an extension online via the ICA website or at the ICA visitor services center. Thirty-90 day extensions are typically given depending on the visitor's purpose in Singapore:
Visitor Services Centre, Immigration & Checkpoints Authority, 4th Storey, ICA Building, 10 Kallang Road (Next to Lavender MRT station)
The ICA also grants long term extensions of stay for the following reasons:
As requirements, forms, and supporting documents are vastly different for each category and sometimes change, visit the ICA website for specifics.