I go into mourning if it's over 1 inch.
Ideally, I want to trim a 1/4 inch every other relaxer. My hair is naturally fine/thin and I wear it out daily, so trims really help my ends stay fortified. But unfortunately I went most of the year with no trims, trying to growing longer. But I felt the ends were thinnish, so I snipped about 2.5 to 3 inches a couple weeks ago. Because this was the thin bottom of the hair, I didn't expect it to make much of a difference. But to me that took me back to the common length again (avi). Nothing special -- and that made me sad.
Oh well, I vow to trim regularly (every other relaxer/every 3 months) forever so I won't have to do anything drastic.
Edit: Oh and surprising, the hair's whole personality changes with a trim ... it hangs differently, looks different, and has a whole new aura. It takes some getting used to -- especially if you liked the "personality" of the old hair.