lengthen out
To become longer. You'll have to wait for your hair to lengthen out a little bit before you can really style it. I hate how dark the mornings and nights are during the winter, so it's always a relief when the day lengthens out in the spring.
lengthen out
to stretch or grow longer. The days began to lengthen out and we knew summer was upon us. As we approached the end of the trail, the distance seemed to lengthen out.