Can she apply for visa if not working and how long will it last? What are the required documents?

It is possible to try to apply for a multiple entry valid for 6 month, but it is not assured that she can get it in the end. She is an Indonesian and she plans to apply in Singapore, so it may not be easy to get it in the end.

Usually, it may be easier to get a single entry Tourist L valid for one month for those applying in a third country.

If she really needs to stay that long, a detail travel plan is needed for the application. Besides, her proof of legal status in Singapore as well as valid passport, an application form, a recent taken photo, round trip tickets and hotel bookings are needed for applying.