The K-1 Visa processing timeline varies greatly from case to case.
Aside from that, the processing was also severely impacted by the prior Trump Administrations immigration restriction and now by the pandemic.
The best-case scenario is that the processing can take between 6 to 9 months and that is assuming that USCIS does not issue a request for evidence, or your Fiance is not from a country that USCIS considers high-fraud (Nigeria, India, Ghana, Morocco, Yemen, Kenya, Pakistan, Philippines, China and Mexico) or have any criminal antecedents.
To ensure that your case is processed in a timely manner you should file a strong case with a lot of evidence proving your relationship is bona fide and explaining any prior immigrant visa petitions filed by the US Citizen.