For as long as you rent the plot, as well as if you subsequently transfer the lease agreement to another owner.
Leaseholds offered by EuroParcs are rent-dependent leaseholds. As the name implies, the leasehold depends on the lease agreement. As long as you are a tenant of the plot, the leasehold continues to exist. If the lease agreement ends, the leasehold ends. The leasehold itself cannot be terminated.