Pretty much as soon as we joined. But especially once it became clear that Monnet's idea of a Federalised Europe was likely to be the final destination.
Worth having a look at the Referendum Party and James Goldsmith (who is an very interesting figure).
I did not realise that the idea of a Brexit referendum had been around for such a long time and is not just the idea of Tories/UKIP.
https://www.bbc.co.uk/news/uk-politics-49753420