Most what
Cartageno said is true, but the formular
changed some versions ago. 1 point in fitness takes away 1 hour and 2 points in sexperience also take away 1 hour. With low libido and 100 fitness you need 256 points in sexperience to fuck every 24 hours (You could of course go higher, with 280 the threshold is at 0).
The best way imo to gain sexpierence, are the sexperience books which give 20 points (after reading 10 times), so 13-14 books should be enough. (maybe even less depending how much sex you have... ^^)