Noone can tell you - you'll have to find out yourself. It depends on what music you hear, your equipment, your ears/training in hearing artifacts etc.
There's no simple answer. It's a matter of statistics. There are real music samples where everyone who's not deaf can hear what's wrong with a 192kbps file. The lower the bitrate, the more likely it is to run into noticable differences. Problematic sounds are e.g. complex high frequency sounds like hihats or cymbals, or applause (live concerts). In my exerience 128kbps (see sticky thread about recommended lame settings) is good enough for portable/car use, but in a PC environment where space is cheap (CD-Rs etc.) I would rather back some mp3s up to CD-R and delete them from HDD than transcoding them to save space.
