How many years approximately will it take before nuclear fusion is an efficient and usable source of energy?

Question: How many years approximately will it take before nuclear fusion is an efficient and usable source of energy?
- Keywords:
  energy,
  fusion,
  nuclear
Asked by Joe to Ed, Kerrianne, Nina, Oli, yoyehudi on 8 Nov 2017.
- Oli Wilson answered on 8 Nov 2017:
  
  Too many, if you ask me. It would really be beyond amazing to have such a massively powerful, clean and cheap source of energy available to power the world – just think of what we could do and the advances we could make!
  To put a figure on it, I had a look on the website of the Culham Centre for Fusion Energy (http://www.ccfe.ac.uk/Fusion_power.aspx), who are part of the 27-nation European network trying to make that dream a reality. They say that fusion power might be contributing to the national grid by the 2040s or 2050, but there’s a huge IF there – by 2050 *IF* they have the funding to keep up the research. With the UK set to leave the EU with brexit, there’s much less security of scientific funding for things like this, so it might end up being much, much longer before fusion can help us meet our energy needs. What a shame.

Question: How many years approximately will it take before nuclear fusion is an efficient and usable source of energy?