Some ads say, "Learn English in five minutes a day!" But the numbers do not match. A U.S. government school for diplomats says English speakers need about 2,200 class hours to use Japanese at work. That does not include homework.

With five minutes a day, 2,200 hours would take more than 70 years. With one hour a day, it would take about six years. So the amount of time really matters.

Ads promising fluent English in five minutes a day are common, but the numbers tell a different story. The U.S. Foreign Service Institute, which trains diplomats, estimates how long English speakers need to reach a professional working level in other languages. For languages close to English, such as Spanish and French, it is 552 to 690 class hours.

For Japanese, Korean, Chinese and Arabic, it is about 2,200 class hours, plus about 17 hours of self-study each week. These figures are for English speakers learning other languages, not the reverse, and the learners are adult government employees in intensive classes. Even so, the estimate is a useful guide. At five minutes a day, 2,200 hours would take over 70 years, while three hours a day would take about two years.

Advertisements promising fluency in five minutes a day collapse under simple arithmetic. The U.S. Foreign Service Institute, which trains American diplomats, publishes estimates of how long native English speakers need to reach professional working proficiency. Category I languages such as Spanish and French require 552 to 690 class hours, whereas Category IV languages, including Japanese, Korean, Chinese and Arabic, require about 2,200, with a typical week combining 23 hours of class and 17 hours of self-study.

The figures are often misquoted. They describe English speakers learning Japanese rather than Japanese speakers learning English, exclude independent study, and assume adult government employees in intensive classes. Even so, as a rough benchmark they are sobering: at five minutes a day, 2,200 hours would take more than 70 years. How the hours are arranged also matters.

In a 2007 study, Serrano and Muñoz found that adult English learners progressed more when the same number of hours was concentrated into an intensive course than when it was spread out. Memory research, by contrast, consistently shows that spaced review improves long-term retention. In practice, that means combining blocks of intensive practice to build skills with regular spaced review to keep what has been learned.