The timeline below show the European fusion plans for a DEMO plant which would generate 500MW of electricity. The timeline of DEMO is dependent on the success of 6 milestones from ITER. A particularly essential milestone is the validation of the Tritium Breeder Module of which there is considerable doubt, with some experts predicting that ITER will not provide an adequate demonstration of the complex systems required to re-cycle tritium in a real reactor.
Several countries have independent plans for a prototype reactor based on the TER program and design basics. The Chinese plans are illustrated below and show after ITER, a first prototype reactor of, 200 MW, further validating certain systems, followed by a 1 GW reactor. Even with this extra step the timescale indicated is similar to that of Europe.
In the US many scientists and engineers consider an extra R&D facility, FNSF, is required between ITER and a DEMO fusion reactor prototype. Neither this facility or the US DEMO is funded as yet.
Introducing such a further step in the schedule clearly would delay the DEMO reactor by decades.
Predicting when a prototype will actually be in operation is close to impossible. The plans above for Europe and for China project operational devices around 2050, but already ITER is late compared to these plans. As discussed on the page Challenges for Nuclear Fusion Reactors , there is an enormous challenge to make a tritium breeder system and the US considers the FNSF is necessary step before launching construction of the US DEMO. This would put a prototype fusion towards 2080. With so many uncertainties, no schedule can be considered reliable more than 50 years in the future.