There are two answers - one where the answer to x is 4 years, another where the answer is 10 years.
If one constructs a dwelling or commercial premises, the period for which the local authority has to inform the owner of the land he has built illegally is four years.
If one changes the use of existing land from, say, residential dwelling to business use, the period of time after which nothing can be done to prevent the change is ten years.
I suspect in this case, ten years has to pass - I suspect the use has been changed from agricultural to commercial, not a new structure built. Talk to the local council planning department - now.