You can buy bitcoin when the price reduce. I think you can buy now since the price is still rising. You can sell your bitcoin when you are already contented on how much you earn from it. It really depend on you on how you will handle you bitcoin investment. Just be wise and read news about bitcoin. News that will affect the price of the bitcoin.
Now is the time to sell your bitcoins since people are dumping their altcoins for bitcoin because of the hard fork that will happen. I will suggest you to sell now and rebut later when people have done dumping their bitcoins for their altcoins back.