Nope there is no rule for atleast 1 year, it very much depends on the mood of the officer making the decision.
Sorry for ur bad lauck. I am keeping fingers crossed for my in-laws, my parents got 10 yr multiple. But they are both in govt jobs and have travelled quite a bit. Whereas my in-laws are retired/housewife etc.
Maybe if ur parents come back early this time next time they will get a better duration.