This article here from the BBC looks at how long it can take immigrants to the Uk to learn 'fluent' English.
Figures from this article say that '726,000 people in England and Wales said they could not speak English well, and another 138,000 said they did not speak it at all.'
Factors impacting the time to learn English are quoted as:
- Age of the individual
- Number of languages already spoken
- Literacy and education in mother tongue
- Exposure and interaction with native English speakers
- Access to classes