Hi - thought you might find this research interesting reading.. here's their findings in short:

The probability of conception ranged from 0.1, when intercourse occurred five days before ovulation, to 0.33, when intercourse occurred on the day of ovulation. The probability of conception was highest with daily intercourse (0.37) and dropped to 0.15 with untimed intercourse once a week. Increased frequency of intercourse did not lower fertility, disputing the belief that frequent intercourse will decrease the potency of semen.