You need to define how much risk you want to take as your stop.
For example, lets say you sell out of the money calls and puts for the SPY which will expire in say Sep 18.
We know one side of these contracts is in fact going to expire worthless.
Now we need to limit risk on the other side.
This can be done a few ways.
1) Define how much heat you will take on your position if you are wrong. For example, close the position if it goes 3x 4x or 5x against you. For example, lets say you expect a $ 1,000 profit from the sale of the calls, and now you are looking at a $ 4,000 loss if you close the position today, if you are willing to take a $ 5,000 loss, then nothing needs to be done, but if you are only willing to take a $ 3,000 or $ 4,000 loss, then you need to close out your position.
2) If the position has gone in your favor, you can then buy further out of the money options which then limit your risk of a sudden reversal.
3) You could buy or sell short the actual index to limit some of your risk.
4) You can define actual price points where you theory is wrong and you should close the position. For example market rallies up to 920, you expect that will be the top within 5 points so you sell some calls with the stop loss at 925. Market then heads to 926, you kill the position if there is still many weeks left before expiration. However, if say its just a few days with no major economic reports, you may just want to wait it out.
