What % you sell depends on your risk tolerance and initial investment. For example perhaps you sell 50% after the price doubles, then you'll have your money back and hold half of your current moneroj. I think a good way to go about it is to decide on the amount of XMR you want to own long term and then buy more, perhaps 2x what you want to own. Then you diversify as the price goes up until you have sold the exceeds, recovered your initial investment and still hold your preferred long term position.