As already stated, 14 days from the date of the 'notice of vacancy' which is issued by the Returning officer / Electoral services of the local authority.
If your query is at PC level there is no need (nor ability) to get involved.
Clerk notifies Electoral services of the vacancy arising, Electoral services issue the 'Notice' (which should be dated and state the 14 day deadline for a request for election, Electoral services will issue a further notice stating whether (or not) there were sufficient qualifying requests for an election, if "yes" details for election and if "no" informing PC that cooption may occur.
The only action for a PC in that chain of events is for the clerk to inform the Electoral services of the vacancy having arisen and then to publish any notices provided by the Electoral services office.
Have you received confirmation from Electoral services that an election is required?