Building must be complete within five years of the date of grant AFAICR.
You can seek to have this extended if you have completed substantial works.
I'm not certain what the absolute definition of that is, and its not a certainty.
Compliance with conditions through enforcement action is limted ot 7 years IIRC.
This may be unlimited in the case of the use of land, i.e. a mining or extractive use.
I'm not sure this has been tested in Court, because I read recently that the Statute of Limitations regarding Tort is 6 years.
For Land it is 12 years - as in continuous occupaton and sole use and fencing it in leading to making a claim for adverse possession of land, unless I misread it totally.
Normally mf1 is on the ball, but I understand that properties can be sold even if conditions have not been complied with.
It all comes down to risk and profit.
I think there is a duty of care on the vendor's solicitor to disclose matters like this to the purchaser's solicitor.
But the existence of a non-compliant matter in and of itself doesn't mean it will be impossible to sell on.
It will make the purshaser's position that much stronger.
It may tie down the vendor into giving some outrageous assurances.
For example, that he/she will procure compliance or a revised permission or forfeit money.
FWIW
ONQ