If you go by the ads for some language learning apps, you can “have a conversation in a new language in three weeks”.
But the experience of most Australians when trying to learn a new language is more likely to resemble that of our prime minister who, a few years ago, wrote:
Learning any language at school is…difficult because there simply aren’t enough hours in the school calendar for most students to achieve any real facility – as many Australians have discovered when they tried out their schoolboy or schoolgirl French on their first visit to Paris!
The time it takes to learn a language depends on what you mean by “learning a language”. If your definition is being able to order a “café au lait” or ask for directions to “les toilettes, s’il vous plait” on your next trip to Paris, three weeks is perfectly realistic.
Source: The time it takes to learn a new language depends on what you want to do with it