Long ago. Looong ago it already started look like a very bad, and, to be honest, politically and economically unsustainable idea. But for the general public it began to be increasingly evident 1) politically when the so called Constitution (later to be replaced with Lisbon treaty) was failed several times 2) economically when the specter of communism in its modernized guise again started haunting Europe: Germany and some other countries produced and paid, Greece, Portugal, Spain, Italy, Ireland etc just borrowed and consumed.
The problem is that EU, in the form it is now, is a kind of Union of European governments, not of European Nations, and it's not controlled by peoples, that's a sad reality that goes fundamentally against the political tradition and self-consciousness of the nations 'misunited' under the EU flag. In practice it is a purely bureaucratic structure, and maybe it would be OK IF it would not be sold at the same time as a kind of 'true product' of the total unification of peoples, which could be possible only and only under the condition of true democratic control. It is just a club of the governments that use this machinery rather often to avoid control on the part of their respective nations. So not only is this 'Federation' undemocratic, but also it makes less democratic in practice those nations included in this sweet project.
Economical problems partly emanate from the political ones, cos you cannot make a unified currency zone (eurozone) without making a unified fiscal zone. But the latter, obviously cannot be done without a serious 'blurring' of the sovereignty of the participating states, that's, without some kind of tectonic political merger with the formation of some kind of united states of Europe. From the very first it was obvious for many many critics that you cannot essentially deprive countries of their right to print its own currency, giving them instead the right to borrow without restriction the new common currency, but at the same time allow them to control their budget totally on their own. You don't have to be Einstein to understand that every Government in those poor countries will just borrow as much money as it wants just to stay in power its 4 or 8 years, shifting the burden to the next people.
Change the EU in a better way and then it might be worth for other nations to be part of it