When you are fertile (usually the 5 days preceeding ovulation) the sperm can live for 3 days inside you. So even if you have sex 2 days before ovulation, you can still get pregnant if you don't have sex again.
12-16 days before your next period start having sex every other day for the best chance.
To answer your question yes. There are only 72 hours (three days) in which the egg is ideal for conception. Your best bet is to start just before you ovulate, then have sex every other day for a week or so. Even then you still might not conceive for a few months.