average time

This is about what I could find...

According to various surveys, 40 per cent of British couples make love more than three times per week, 35 per cent make love once or twice a week and 15 per cent make love two to three times per month. Nine per cent of couples make love less than this or not at all.

The duration of a relationship is also significant. Other surveys have found that over 50 per cent of couples together for under three years make love more than three times per week. After four or more years together, only 25 per cent maintain this frequency.

http://www.tiscali.co.uk/lifestyle/healthfitness/menshealth/part1_7-1.html

A recent study shows that married couples in their 20's have intercourse an average of four times per week, whereas that number drops to one time per week for couples in their sixties.
http://www.womenshealthmatters.ca/facts/quick_show_d.cfm?number=181

Good marriage counselors will tell you that there is no such thing as "normal" frequency. Whatever is right for a particular couple is fine, be that three times a day or once every three months. The studies Linkat posted show a higher average frequency than those I am aware of. In addition, most couples change frequency at different times in their relationship.