I did this as a drill when we were in Maui, a couple of years ago. My husband had gotten about 20 feet or so from me. I waited to the bottom of an exhalation (since when you find out you're out of gas is most likely when you begin to inhale) and spat out my reg. I started swimming to my husband. I could not get his attention, because he was not looking at me, and he did not hear me banging my tank. I first discovered that I had to swim UP as well as over, because, of course, with my lungs empty, I was negative. This increases the workload significantly. By the time I got to him and got his attention and got his reg, I was not very happy. This told me that 20 feet was TOO FAR.
Paradoxically, most people feel more relaxed about opening up buddy separation in excellent visibility, and this is where you need to stay closer, because you have no very effective way of attraction your buddy's attention.