It all depends on the situation, freshwater or seawater? what's the viz? Task loaded or rec. diving? In freshwater, lakes, rivers farm ponds and stock tanks, I would never get out of the range of the viz. As for Saltwater and Mines I would not get beyond 1 breath, that is how far you could swim on one breath in an emergency situation.
As for signaling distance, that is not an option, as you learned in O/W it is to say, difficult at best to determine the direction from which a sound came. On the other hand, with a reliable & proven underwater communication unit the distance could be greater.
If you are fortunate and dive with the same buddy all the time you will get to know what distance you will be comfortable with. Also if you dive with the same buddy all the time you will start to think alike and the distance will become automatic. I know this sounds strange but I'm sure others will back me up on this.
I am always aware of my buddie's position in relation to me, and that doesn't mean rubbernecking to keep him in sight, sometimes you can just sense it, catch him out of the corner of your eye, see his bubbles in your field of vision etc, and if that position gets out of range then I signal the "Buddy Up" signal and continue on.
Just my .02
ID