Different for different people but for some 12mm would be a huge difference and could be enough to ruin hips and knees etc. over a long enough period of time. At a minimum most will likely notice the difference in offset of a few mm but again it depends on the person. As an example I'm in my mid 50's and the older I get the more important the perfect Q-factor becomes. I find that over 1mm of offset in either direction will cause some knee pain before I even ride 5 miles. When I was younger, not so much, but even then over 5mm off was noticeable. Best to try it yourself to get the answer you need.