I just learned from this question My professor skipped me on Christmas bonus payment that in some cultures/degree-programs, PhD students get bonuses.
I had never heard of this. However, I'm most familiar only with natural science PhD programs in the US.
What countries/cultures and programs are bonuses common?
How big do these bonuses tend to be compared to salary? (if there's a rule of thumb, or commonly the result of a collective bargaining agreement as stated in one of the comments to the above question)