That depends on the virus load you’ve been exposed to, so on the incubation period, on how soon you’d start manifesting symptoms (or maybe also on how soon symptoms would either progress or abate if you appear to have symptoms).
So for medical professionals, who get exposed to a lot of the virus, the self-isolation period is 7 days. For people who maybe were exposed to one person with mild symptoms, it’s 14 days.
Because if the latter got infected, there would be very little of the virus in their body and it would take some time for the virus to replicate into great numbers and overwhelm the body.
Does that make sense?