I've been knitting forever and I rip things out all the time. There is no shame in ripping a row, half a project, or to nothing. I rip because I've made mistakes, don't like the gage I'm working at, made it too big, made it too small, etc etc etc.
Heck, I finished a hat with no errors, sewed up the seams, finished the ends and wove them in, then RIP. The size was just a tad too big, and I wanted to change the pattern. After it was completely done. Hours and hours of work poof just like that. Of course, when I finished it the second time, it was exactly what I wanted, and I had the pattern memorized by then and could work it in my sleep.
I'm working a a sweater now that I had gotten about 10 inches into when I decided I didn't like the gage. The fabrick was too stiff. Everything was measuring fine, I just didn't like it. So I ripped it out, recalculated EVERYTHING to the new gage (I'm not working off a pattern), and got going again. Now I'm nearing the home stretch, no worse for the ware but a few hours of learning time.
I've been able to work the same yarn a few times. It depends, so far as I've noticed, on how much you twist/untwist the yarn while you work and after you've frogged it. And there's no consistency; it depends on you and the yarn. Some yarn I know I could rip and reuse a dozen times, some yarn I can barely rip in the first place. It has to do with how fuzzy it is, usually, and how tightly its twisted in the first place. I don't think there's really any one answer to that question.