The decision of when to cut your losses is one of the toughest questions in trading. Generally, some people will tell you never to lose more than ~2% of principle in any trade; however, it's not necessary easy to follow that in practice, especially when trading highly volatile markets.

Discipline is the key to trading and you need to learn from your loss. Never invest more than you can afford to lose, have a proper trading system and adhere to it firmly, learn to hedge (!), and don't be greedy.