I grew mine out when I was about 38, I think. It took several years before all the dye was gone. It looked worst for the first few months where you have a really clear line on top of your head. As it got longer the line was less clear and it looked better. I think a lot of people who start growing it out and then go back to dyeing never get past the first terrible looking months.
I think on reflection I could have kept dyeing my hair for a few more years. I felt I was a bit young to have grey hair when it was all grown out but I wasn't prepared to start dyeing again and have to go through the whole process again. Now I am in my mid-forties, I feel my grey hair suits me better.