So many myths to bust. So little time.
Whether the relationship is one of employment or self employment is a highly complex issue based on a large number of factors considered in the round. Length of service is not a significant factor, at least not in isolation, and where it is a factor of any relevance at all, there is no set time. There was an occasion when "red" Dawn Primorolo (back when Labour were in power) opined that 3 months was relevant, but that has never been upheld in courts or tribunals, nor does it appear in any statute.
Nor is whether the "employee" is otherwise registered as self employed in other work a deciding factor although it can be influential.
Nor is it simply a matter of choice. A popular myth is that the parties simply have to agree between them that it be treated as self employment and job done. Not so.
And the very LAST place that I would go for independent advice is guidance published by HMRC, who have a vested interest in having arrangements classified as employment.
That said, HMRC do provide a very useful "employment status indicator tool" on their website. Go to their home page and put that in their internal search engine.
Even then, HMRC state that they will not be bound by the outcome, although provided that you do it properly you are somewhat protected from penalties and back duty assessments if the result gets overturned.
To be frank, you may be best advised to go to a professional and pay for an opinion.