Like someone said, if you are ever stuck on a desert island and can trade only one options strategy, it should be a vertical spread. Dan from Theta Trend has a trend-following strategy executed via credit verticals which really strikes a chord with me. So here is a picture illustrating what to expect with those (click it to enlarge).
This is a 110-115 bear call spread open with 60 days to expiration. The P/L figures assume this was open with the underlying at 100 (making the short call about 13.5 delta at the time). Implied volatility is assumed constant at 20%.
I see two notable features here. One is the flat spot in the far right corner of the surface. It shows how when the underlying stays far away from the spread strikes, most of the profit potential is realized long before the expiration. Tracing the 100 price level in the picture on the right we can see how it flattens in the final 20 days or so. And if the price drops further to 95, most of the juice is gone in the final 35 days.
Another feature is how at the price level corresponding to the short strike the decay starts off very slow and accelerates as the expiration approaches. And at price levels below the short strike but close to it, such as 105, it is a combination of both: decay accelerates as time goes by, but most of it is over by the final week.
These features are similar to how time decay of single options works, which is of course only natural. So the main takeaway here is that trading a directional time decay system like Dan’s, if the price does move in your favor, your position is going to end up in that flat spot long before your options expire. Which is the time to book the profits and take risk off the table.