The parental leave policy for graduate students can be found here. Graduate students receive “up to eight weeks of [fully paid] leave for the primary care giver following the birth or the adoption of a child”. Graduate students are only eligible for the additional 8 weeks at 50% pay in very specific circumstances (working in a non-academically required position for 175 days before the first day of leave, more information here.)
For information on parental leave for faculty, take a look at the following chart and timeline prepared by John Jaenike:
A PDF version of this information can be found here. For the most up-to-date information on faculty leave policies, see the Leave administration page on the HR website. There are also university policies on modified duties for faculty members that are new parents, part-time appointments in the year following birth or adoption, and postponement of the tenure clock for faculty with a new child. See more information in the faculty guidebook here.