久别情疏-------Out of sight，out of mind.

例句：
A: After enlisted, he lost track of his girlfriend.
服兵役后，他就与女朋友失去联系了。
B: Out of sight，out of mind. Haven't you heard of it?
久别情疏。你没听过吗？

同义词：
Far from eye, far from heart.
Long absent, soon forgotten.
Seldom seen, soon forgotten.