A good rule of thumb regarding fuel additives in general is that they will perform best during extended periods of driving. A highway-based road trip would typically be ideal, but running at high RPMs would not be necessary. As for the longevity in the gas tank of Berryman B-12 Chemtool Fuel System Cleaner (part #0116), it should remain effective until fully consumed in all modern automobiles. In most marine applications, 0116 should be added the day of use.

What are the most important factors in play here? For example, based on your response, I imagine some factors would be continuous engine running time (from "extended periods of driving") and perhaps speed or frequency of acceleration/deceleration (from "highway-based road trip"). Since the recommended practices are considered "best" and "ideal," I am curious how significant the difference in effectiveness is when compared to the cleaning from short trips or even simply idling the car with the product. Thanks for the prompt response!

The most important factor with fuel additives is the amount of time the vehicle runs. The longer it runs, the more continuous contact time the chemical has with the deposits and the more thorough the level of cleaning will be. Highway driving can be expected to afford slightly better cleaning than start-and-stop driving, which should be a bit better than simple idling. Short trips will be the least effective. As for the significance of these factors, that's not really possible to quantify because there are just too many factors. The degree of buildup, the nature of the deposits (gum vs varnish vs carbon), and the location of the buildup (injector tip, intake valve, piston crown, etc.) all influence the process.

Interesting. Practicality aside, wouldn't idling through the entire tank have the engine run longer than highway and start-and-stop driving, thereby affording the most thorough cleaning?

Contact time is just one of the relevant factors. Operating temperature is also important because some classes of chemicals, such as PIBSA-based detergents, require heat for activation. Some components—catalytic converters, for instance—perform better under load as opposed to idle, too. There is rarely a perfect scenario in the field, but highway driving is generally the best fit scenario when taking into account all the relevant parameters.