Kitros: I'm not positive, so don't take my word as the truth, but I believe that if you buy a share as a member of the public after the company has just IPO'd, then you can sell whenever you want. If you got in before the IPO, then I believe there is a waiting period, then you have to apply to the SEC so that you can sell your shares within a certain time period.
Again, I'm not positive of that, you'll have to check up a reliable source to be sure.
Remember, all corporations have shares... the IPO is just the initial public offering. Once the offering is made PUBLIC, I don't see how they can restrict your purchasing and selling of shares to a certain time period (unless you somehow wanted to purchase 30% of a company from its available public float, however, I doubt many people here are going to do that).
However, again, check up on a reliable source (broker, regulations guide) before making any decisions.