That depends on the water temperature. During the summer, when the water is warm, going 20 days without testing would be an obvious problem. But in the winter, when the water is cold, it is often quite reasonable to go that long without testing the water. Cold temperatures slow down all of the changes that might cause problems. Just how long you can go without problems depends on the actual water temperature. Generally, when the water is below 50 degrees you can go at least a couple of weeks without worrying about testing.
The colder the water, the higher you can allow the PH to get. For roughly every 11 degrees colder the water gets, you can allow the PH to rise about 0.1 higher than you otherwise would allow it. Normally the PH will shift by about this much on it's own as the water cools.
In extreme situations calcium scaling can happen very quickly, say over a couple of days, but conditions would need to be fairly extreme for scaling to proceed that quickly.
