Ummmm…I didn’t ask if how many times you had it your head with a throw. Sorry if you took offense. I was just joking. Apologies.
What is so favorable about playing a string until it breaks?
Throwing upwards isn’t the only way you would hit yourself. If you throw downwards with enough force, it will compress the string and the yoyo will “rebound” in the opposite direction of the throw.
Pretty unlikely, though. 
I was mostly just poking fun at how hard you would have to throw in order to break a string. Pretty hard. Hard enough that it could rebound!
The more likely scenario is that if you’re somebody who rips their yoyo that hard, all it would take is one snag for that yoyo to pivot unexpectedly and hurtle at your head and cause a near-death experience avoidable only through bullet-time and catlike reflexes!
No need to buy strings so often.
I do have a pretty hard throw. And very time i play 3a, I never break a string. They last from contest to contest and i only replace them then. I could probably play 3a strings for a year before they’d break lol because the strings are so short. And I have had a few “near death experience” like you said. But i manage to escape. One thing I’ve learned. Never leave a triangle (knot) in your yoyo.
The difference between when when you should replace a string and when it breaks shouldn’t be more than a day or two unless you are practically non stop throwing
With a standard poly string an hour or two a day every day, same string on every yoyo, it would take a few weeks to snap it.
I would know because that’s what I did when I was starting out :
No matter what, when you bind there is SOME friction going on. When your yoyo is tilting, even without a string-eater, there is SOME friction on the wall…
Inevitably this is going to wear down a string.
Without being able to put my finger on exactly “why,” I don’t think that sort of wear and tear is in the spirit of the original question. That sort of question would be more like, “How long does it take your string before it’s worn out to the point of snapping.” The original question seems to imply that it’s the repeated force of the throw rather than fibres wearing down through friction and grime.
Lol my string just broke at the loop on the finger.
Now this actually makes sense to me! Here is a cluster of fibres, and they’re “kinked” repeatedly at more or less the same point. There is likely also friction as the string slides a little bit in its slipknot. That’s going to wear a string down at this point.
If anything, it seems MORE likely to me that a string is going to break at the finger loop than at the axle. Unless you have a string eater.
I use a finch-head knot for my slipknot… which has the unexpected side benefit (I’m only doing it so that loops don’t go flying off my finger!) of lessening the type of wear and tear described above!