Crystal Gayle was able to preserve her hair without much thinning at the ends to ankle length until about the age of 50.

I think the answer is quality. If the ends start to look inconsistent with the mane, it might suggest a trim to even out the look.

For me, I have always been enamered with bottom of the "bum" length with a preference for mid-calf to knee length.

For you, what you feel comfortable with is what you should choose. I envy those who can grow a mane to their ankles.

xoxox