This is a hard question to answer and to be honest there probably isn't a right or wrong answer. The place you should be looking for the answer to "when to exit" is your strategy, but by the comment and that fact that you are new I'm guessing you don't have one. If you take a look at my strategy development page
it will give you some kind of idea of what you should be including in a strategy if you want to try an develop your own of course.
If you already have a strategy in place for entering the trade but are looking for better exits then it really depends on your entry strategy.
If your just after an exit strategy then here are a few to help you out.Risk/Reward ratio exit
If you know what your initial risk is(distance between entry and stop level) then you could aim for a ratio of that risk as your reward say 1:3 risk reward. Therefore if you risk £10 on a trade you would look to make £30. This would give you an exit price and is a good strategy to use. I would suggest aiming for a minimum of 1:3 though as this means you can get 70% of your trades wrong and still make money.
If like me you don't like to cut your winners short with a risk reward ratio you could use a trailing stop exit. Therefore as the position moves into profit you move up you stop inline with it locking in profit along the way. It's important to note that you should only ever move your stop in your favour. Never move a stop back the other way. To trail this stop you could trail at some % of price away from the current price or use an ATR trailing stop or something else it's really your choice.
You could also use moving averages and exit when price crosses said moving average.
I'm sure by now you've realised that the question you asked really raises more questions than answers. I would suggest having a read through my strategy section & some of the beginners guide and possibly some of my technical analysis section(although I need to add way more to this).
Sorry I can't be more help. If you have any more questions feel free to ask.
May the markets be with you!
The Spread betting beginner