That's a question YOU need to answer. There is inherent uncertainty in the market. Personally, If you have not already I recommend you read
Market Wizards. This should give you some insight. There are many approaches to this and it depends on the time frame you trade and what type of trader you are and your ability to delay gratification and not panick. There is no pat answer to this and I would be highly wary of anyone who claims to know 100% that answer. The real question is do you have a method of not allowing losses to take over? It is the losing trades or the failure to contain them that destroys trading accounts. Everyone has losers the question is are you able to be good at losing. The better you are at losing the more money you will make.