Regs don't state max distance as each ring with spurs must not exceed 100 Sq. metres area, this would effectively mean 10m X 10m so is self limiting. If an extra long run were required it would have to be as a radial when the size of cable would be set by the volt drop calcs.
If you can give more info as to what you have in mind maybe an answer can be found, regards SPARX
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