MR is famously hard to get started in, for a career that pays relatively little for the first few years (note that salary's now much better for senior levels than it was 20 years ago). Many will get into the industry via a graduate training scheme at one of the big agencies - this will feature a formal induction course of weeks or months, and give new recruits a good peer group to learn with. However an increasing number of small-to-medium companies now have their own grad programs, and there have always been openings for fresh graduates in a wide variety of agencies - just fewer of them than in the big companies, and until recently much harder to find.
It can take around a year to work one's way through being a trainee and move on to the next stage, Research Exec (UK) or Junior Analyst (US - although note there is a much wider variety of title in the States at this level than in Britain). Those who have done internships or placement years may have a head start and get to RE level in less than 12 months, and those in very formal set-ups or particular company circumstances may find it takes a bit longer. Don't forget we're not just talking about job titles here - in rare cases quite good people can still be called a 'Junior Researcher' even when they have 3 or 4 years' experience - just depending on the habits of the employer.
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