If you have a long-term vision, then buy as much BTC as you can. There is no right time to buy it, Keep investing at regular intervals to have a great return. But if you are short-term investors then keep a keen eye on BTC movement, try to buy at dips. Don't invest all your money at a time, try to average the price so that you have best purchase rate.
Yes long term investment is too much profitable and very little chances to lose money, but i think that in present time you should wait for some time tell the hardfork. because there also chances that the price of bitcoin dump a little after hardfork. so you will get some chances to buy bitcoin in a low price.