In 1997 Padian et al published the results of a 10 year study on heterosexual transmission of HIV in northern California.3 The data were divided into two parts, cross sectional and prospective. From the cross sectional study it was estimated that infectivity for male to female transmission is low, approximately 0.0009 per contact, and approximately eight times more efficient than female to male transmission. Using this estimate of male to female transmission, it would take 770 or 3333 sexual contacts respectively to reach a 50% or 95% probability of becoming infected. If sexual contact were to take place repeatedly every three days this would require a period of 6.3 and 27.4 years respectively. Based on the estimate of female to male transmission by Padian et al it would require 6200 and 27 000 contacts and a period of 51 and 222 years, respectively (table).
http://www.healtoronto.com/nih/