UFLS (Under frequency load shedding) in Islanded generation

What is the typical time delay used in an under-frequency load shedding scheme for an Islanded generation, and what criteria are used to determine this time delay?
1. Should the under-frequency load shedding (UFLS) scheme be combined with a rate of change of frequency (df/dt) along with a time delay?
2. Or should it only use under-frequency relay settings with a time delay?
3. This UFLS is backup to the FLS (Fast load shedding scheme in PMS)
4. The UFLS is configured in the 11kV bus tie relay.
5. How is it decided that group-1 is to be set at 58.5Hz, group-2 at 58Hz...?