i.e. do the holders of PR need to stay in Singapore some minimum amount of time every year?
PR status for Singapore can be lost if the person travels outside Singapore without a valid REP (re-entry permit), or if the REP expires while the person is overseas.
You can apply to renew the REP while overseas, but official recommendation is that you do this at least two months before your old REP runs out. There is no guarantee that you will get a new REP, or for how long (up to 5 years, but may be issued for only 1 year).
Documentation requirements depend on the category of PR. There doesn't appear to be an official "time limit" regarding how long you can have been out of Singapore, but this is certainly taken into consideration. Also it matters why you're away from Singapore - e.g. still working for a Singaporean company but posted abroad vs. just having left of your own accord.