Quote from Midas:
Pre determine a stop loss point and when your spread price hits that point take the loss and move on. Pairs trading is a game where we look for a statistical edge and exploit it.
Say you flip a coin 1000 times and you make 1$ every time you flip heads, and you pay .50 every time you flip tails. In the long run you make money. This example would be a risk reward of 1 : 2. T
Now lets say you have probability on your side. Your trade set up might have a 80% probability of going your direction (or a least its history show that to be the case as we never know what the future may hold). In this case you may risk 1$ to make 1$ and still end up ahead in the long run. You will be wrong 2 times out of 10. If you do not take your loss at a reasonable level you will give back everything that you made on the other 8 trades.
If you have a good pair with a high probability of mean reversion and always honor your stop loss, you should come out ahead over time.