The visa can be applied once your parents'd stayed in SG for more than 2 weeks. The form can be obtained from the Info counter, SIR 4th. The processing time will be abt 2 week to 1 month depends on how lucky you are. Once they inform you about the approval, your parents need to go for medical check up in one of the approved clinics (abt S$70 pax).

The longest they can give you is 5-year term (same as PR), but if the passport expires before that date, they will adjust accordingly.

Good Luck!