Depends, at first I want to go at it like rabbits, but eventually even sex can get boring so you need to do it less to make the event more special. (This is assuming you're staying with the same partner for a long period of time)
3-5 but not every day... I noticed that i can't push a proper weight when i work out the day after fucking all night long. It's a pain in the ass, especially on the tougher exercises. But if I'm just jogging or resting the day after -sure.
With my ex we went through a phase of 6 or more times a week, that was good, but ideally I think 3 or 4 times a week is a nice number, keeps you wanting it, doesn't leave you too sore.
If by "sex" you mean each orgasm, two to six per day would be ideal. If you mean each session in bed, it would still be more than once a day, 1-2 per day, maybe 3 on a Saturday. But then the Mrs. and I are still in our early 20s, and both easy on the eyes :)