Is it when you cannot keep up with the dye/re-growth race? When you peer at your roots and think " I only did these two weeks ago and look, I'm silver" 
Or when you think "Hmm, it doesn't actually suit me now, dye looks fake". I don't think my hair remembers its real colour, it'll be interesting at least to see what happens......
Or (which I'm hoping is the right reason) you get your haircut with the intention (honest) of dyeing it but think "Actually, I look quite good"
My hair is short (think Long Pixie) cropped at the back/sides and maybe 2" on the top. Had it cut 4 weeks ago and I was going to henna before we went on holiday but I just CBA.
So, I'm planning to have another crop in a couple of weeks, and see how it looks. If it all looks dreadful, I have some Emergency Henna (but I don't want the risk of orange highlights so that'll be if I really don't like my natural hues)