Det är lite vagt från det jag hittade, men du kan få en vistelse bortom visumets giltighet (förutsatt att du visar bevis på återkomst).
Enligt en Vanliga frågor från deras utrikesministerium (betona min):
How long can I stay in Singapore with a valid e-Visa?
[...]
A holder of a valid Singapore visa who is found suitable for entry into Singapore will be issued with an immigration pass to enter and remain in Singapore. You are advised to check your passport for the arrival visit pass endorsement when in Singapore and take note of the period of stay granted before leaving the checkpoint. The period of stay granted is shown on the visit pass endorsement given on your passport and it is not tied to the validity of your visa.
Dessutom kommer lite mer information om visumgiltighet från denna Singapore turismwebbplats
14. What is the difference between visa’s validity and the maximum allowed stay in Singapore?
Please bear in mind that the duration of each stay in Singapore cannot exceed 30 days, i.e. every time you are entering the country your passport will be accordingly stamped allowing to remain for (usually) 30 days.