I just wrote...

The injection is usually timed to be within 72 hours (which is an arbitrary
number), but it can be immediately following birth or up to one year.

erp... I meant one month, not year. Rh(D)- mothers to Rh(D)+ babies
should be given RhoGAM within 72 hours for best results, but up to 28
days following birth before mother mounts an immune response to Rh(D)+
fetal cells. (Source for that is Understanding Diagnostic Tests in
the Childbearing Year, Anne Frye; a clearing house for summaries and
guidelines to labs for midwives)

Kris