I don't think the question is well posed with regard to the timing. In the formal sense, the ECB is more independent than any other central bank as no single government or national electorate can change the treaty that defines its independence. Unanimous decision to change the treaty in the next 48 months is very unlikely. De facto, much independence has been lost by annnouncing or undertaking SMP, OMT and PSPP purchases of sovereign debt (in a monetary union with otherwise sovereign members.) Governments will continue to expect the ECB to help them out with regard to interest costs. Hence, the ECB will loose many of its current fans once the time comes that it has to stop buying or even start selling government debt to meet its (harmonized consumer price) inflation objective. Lacking the kind of popular support in national electorates that the Bundesbank enjoyed, civil servants in charge of central banking will find it rather difficult to stand up to elected heads of government and finance ministers.