Students often ask how long will it take to see significant improvement.
This is a difficult question to answer since every student is different. The first and most crucial stage is for the student to simply become aware of their flaws. This is a fundamental and essential step. Many students make the mistake of judging their progress by their immediate ability to output natural sounds. In language acquisition both an input and output mechanism exist. In order to achieve a high level of output a sufficient amount of input is necessary. In other words, students shouldn't expect their pronunciation to improve until they have spent sufficient time working on their input.
The ears need to be finely tuned with as much exposure to listening to the natural sounds of English as possible together with a sufficient amount of time spent on shadowing and repetition exercises with the help of a pronunciation instructor. This is a two-step process that involves actual immersion together with hands on correction. This creates a feedback loop whereby the student's subconscious is able to match what they have learnt in the classroom together with the immersive experience of listening to and observing English on a daily basis. The greater the intensity of this feedback loop, the faster the student will adopt more natural pronunciation.
One of the biggest mistakes students make is trying with their utmost effort to speak naturally while ignoring their natural senses. Rather than spending time observing native English speakers and just letting their ears become accustomed to the natural sounds they attempt to manipulate their voices too early during the process.
One of the reasons why children are so good at adopting native-like pronunciation when learning a foreign language is that they rely on their innate human senses whereby they just observe and imitate. Adults have this ability too. It's just that we tend to rely on neural links that have already been developed (the ingrained mother tongue) rather than attempting to establish new ones.