The answer to this question depends on three factors:
The teacher’s competence and work experience
The student’s ability to hear the differences in sounds and the amount of time and effort he/she is putting into the accent reduction process
The quality of training material: books, apps, video courses
Which teachers to choose?
Accent reduction should be taught by a qualified speech tutor (in the past speech tutors were called elocution teachers). A qualified speech tutor knows how sounds are produced in the mouth, the exact position of the tongue, lips and jaw for a particular sound. This is the key. To achieve the best results book a few elocution lessons.
What is expected of a student?
To reduce an accent one must substitute old habits of speaking with new ones which takes time and effort. It is a process. Usually, we recommend mastering a sound minimum for a week. Some sounds are easier and some may take longer to master. For example, /w/ sound does not exist in Russian. It is produced with lips going forward in a tight whistle and requires certain lip muscles. If the muscles are not trained it might take a while to train them to produce an energised /w/ sound. It’s a bit like going to the gym. If I tell you to go to the gym and come with a six-pack after one session you know that it will not happen. It is important to practice regularly, every day for 15-45 minutes rather than two hours once a week.
How to practice? Which books and apps to use?
I recommend that you download professional accent reduction apps Get Rid of your Accent UK1 and Fluent English Speech (British English) or American Accent App and Fluent American Speech (American English).
There is also an Elocution Lessons app to master essential English and a Business English Speech app that contains business vocabulary (both are for British English).
All the above apps have accompanying video courses, Get Rid of your Accent Part One and Get Rid of your Accent Part Two.
It is also a good idea to listen to good RP speakers, copy a section at a time, and record yourself.
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