Every 2-3 days should be fine when TTC.
Also, try to BD whenever you see Egg-white CM - that will keep the spermies alive for longer
There's a method called the "egg meet sperm" plan, for which you need to have some way of knowing when you're about to ovulate (generally OPKs). From your first +ve on an OPK BD every day for 3 days, then rest for a day, and BD again on the 5th day.
Kristina is right, the egg will only live for 12-24 hours, however as sperm will live for up to 5 days in fertile mucus you should be alright BD-ing every few days, you don't have to go at it every day (unless you want to of course!).
Also bear in mind that even if the egg and the sperm make contact, you still only have a 20% chance of getting pregnant. It really is a wonder that the human race survives at all sometimes!
Good luck, and happy BD-ing
Artoo.