They mean 24 weeks in a residential, 24 hour a day program, where you sign an oath never to speak in anything but the target language, and which is largely attended by people in the armed forces whose career will be affected by their performance. The level is probably about B2/C1, and they expect these people to immediately go to work in the language.

English shares a lot of vocabulary with French, so that's a lot less for English speakers to learn.