The official version:
http://pediatrics.aappublications.org/content/120/Supplement_3/S118.1.abstract
http://www.ncbi.nlm.nih.gov/pubmed/16950293
The upshot, obviously is that if you're in the most sensitive part of the spectrum (lowest threshold dose), avoidance for at least that day is a minimal standard.
For other exchanges of bodily fluids, however... longer may be necessary.