Average length of intercourse is 7-ish minutes according to multiple small studies. But, apparently 45% of men finish in less than 2 minutes which is bound to skew that mean!
However, according to a study of 154 couples published in the Journal of Sex Reaearch in 2004, women reported the 7 minutes actual duration as being close enough to their ideal duration of intercourse of 11 minutes as to be satisfying.
Interestingly, they also found that, contrary to what women have been telling men for decades, there is more of a correlation between female orgasm and duration of intercourse than duration of foreplay.
Sources
http://www.newrepublic.com/article/1...x-who-bothered
http://www.ncbi.nlm.nih.gov/pubmed/15497058