The best time to sell your Bitcoin's is when you find the right price.Here the right price reffered to as the price at which you will earn tremendous profits if you sell your BTC's.You can also sell your BTC's when you are in the need of money.I will suggest you to keep holding your BTC's as long as you can because it has a very bright and positive future so it will reach new heights in coming future .So if you want to earn huge profits then you should  not sell your BTC's until you are in the actual need of them.