No need to worry about that, if curtains are the acoustic treatment, because you will never get close.
Here, as always. there are different opinions and taste, like we see in other things, such as.. tuning of frequency response for speakers, just to pick one. Many speakers have a very artistic impression of tonality and frequency response, presumably due to some idea that flat does not sound correct. But, no, flat actually does sound correct, one of few aspects of sound reproduction that has actually been verified in controlled experiments. The same situation appears to be valid for rooms - they need decay slopes within some certain range, and this decay needs to be reasonably consistent across the whole frequency range, for best sound. For all rooms intended for sound reproduction, studio or living room.
Why curtains can not fix everything? Because when you have covered all surfaces, there is still not enough absorption below say 500hz, and too much high frequency absorption. So the room is now sounding too dead, and still has too little low freq absorption.
Curtains can still be a useable tool, especially very heavy and thick ones, like those shown here. In combination with other treatment, for treatement on surfaces such as windows, in rooms where other treatment is not an option.