Weather.com will happily give me temperature predictions for the next ten days, but no indication of how reliable those predictions are. So if I want to plan something for a week from today, or four days from today, I don't really know whether there's much information in the forecasts. My personal, unscientific (and certainly unreliable) experience says there's not, but there's plenty of variation in the forecasted temperatures, even between days 9 and 10, so the weathermen are not simply falling back to the long-term averages for the time of year.
I'm hoping for a rule of thumb here (so my question is different from this theory-heavy discussion from two years ago). Conceivably something as simple as a fixed interval size for each number of days in the future could work, although I could imagine there being low-hanging fruit in, for example, atmospheric pressure or the proximity of the location being forecasted for to water.