When I did my renunciation back in 2018 it took me 6 weeks using the paper submission of the RN1 with all supporting documents. However, others have said that doing their online has meant a considerably longer wait - many did not hear anything for six months and in one case, the renunciation papers were returned back to the Home Office as undelivered. Factoring in six months for the renunciation would mean that it is advisable to do it at latest eighteen months after gaining Austrian citizenship, although my personal belief is that it is best to press on with it as soon as possible, since the last thing I would want is for a long and strenuous procedure to be rendered null and void by a failure to get the paperwork done in time. The complications of the process "lapsing" that Uzbek and Ukrainian citizens I know have told me about are simply not worth the hassle.