When does 20 year TVA (VAT) start?

The 19% TVA on purchase price of a leaseback property was returned if you signed a 'bail commercial'. If the property goes out of lease at any stage then you have to return the TVA on a 1/20th basis (for as many years are remaining in the 20).

My question is when does this 20 year period start exactly ?

1. is it when you sign the lease?
2. is it when you complete the sale of the property?
3. is it when the property first comes available for tourist rental ie. the residence de tourisme opens for business ?
4. other ?

I thought it was when the site opened for business but others think it is when you sign the Bail Commercial (which was a few years before this...) and makes a significant difference on how long is left in the 20 years and how much you might 'owe' the tax office if you go out of lease.

Having said that, many have the view that some of these leaseback sales were a fraud, and thus any attempt to recoup TVA by the French tax office could be considered tantamount to profiting from the proceeds of crime !!

Of course the French tax office might not see it that way ....

thanks