WADR, you are asking more than one question in one, making me think you don't really know much about flints. A good flint in a tuned lock should give you 80-120 strikes before it is " Worn out". That is, its too short to be held securely in the jaws of the cock, with a long enough sharp edge that it will cut steel while still popping the frizzen open timely.
A flint is " TOO THICK " when it no longer is cutting steel, and giving you sparks. That can depend on how thick the flint is to begin with- musket flints are much thicker for instance-- and how long the bevel is to the edge. The longer the bevel, the more strikes you get before the flint edge becomes too rounded ( and dull) to cut steel. In addition to the factors mentioned, the ANGLE OF IMPACT of the flint's edge to the frizzen, as well as the location where the flint strikes also affects how fast a flint becomes dull. Ideally, you want the flint to strike the frizzen 2/3 s of the way up from the bottom of the frizzen when its close. You also want the edge of the flint bevel up, to strike at a 60 degree angle to the frizzen, so that it SCRAPES steel bits from the frizzen in a long, scraping action down the frizzen, popping the frizzen open and out of the way so that the sparks are THROWN into the pan, not dribbled down the face of the frizzen.
The Number of Strikes you get depends on the size of the flint, the kind of lock, whether its tuned, etc. With a good knapped flint- not those cut agates-- ( either the flints made by our own Rich Pierce, using Missouri flint, or the Tom Fuller flints from England) --- you should be getting 25-30 strikes before you need to check the edge for sharpness. Ideal, if that flint is striking the frizzen at the correct angle of Impact, it will knapp itself with each strike, chipping off a bit of the edge to give you a new, clean edge with each strike.
After 25 strikes or so, the flint may be shortened down too much that its not popping open the frizzen, and needs to be moved forward in the jaws. Again, how fast the flint is shortened depends on the lock, the flint, and if the lock is tuned properly. You can't do much with Mother Nature, but you can use a better lock, and you can tune it properly.
The problem with cut agate is that because its cut, by people who have no idea how nature has made that rock( chert family) to fracture naturally, they saw the agates to a gun flint shape anyway they feel like doing it. The cuts do not follow the natural lines of fracture in the rock. A knapped flint has to follow those lines of fracture. Once in a blue moon you will get a cut agate flint that by pure luck happens to have an edge that follows the natural line of fracture of the stone. That agate will produce good sparks, will knapp itself , and give lots of sparks, Reliably. The rest of them will have you cussing in short order. Avoid the high blood pressure, and the cost( those cut agates cost much more than "real " flints) of buying bad "flints", by buying knapped flints in the first place.
Rich Pierce is in St. Louis, so he's about as centrally located as you could hope. He charges a reasonable cost for his flints, and he sends them out to you quickly. You can't ask for better service, or a nicer man with whom to do business. Since Tom Fuller retired, the successors have tried to supply the demand, and by this summer and fall, all the suppliers should be able to provide you with those flints. I bought several hundred of them when I first got a flintlock, over 30 years ago. I still have a few left. But, I also bought some flints from Rich Pierce last Fall, and am glad to have them on hand.
I do have some cut agate flints, bought to try them and to access them on my own. None of the complaints I have heard, before or since, are exaggerated, or wrong. If you want to understand colloidal fracturing, I recommend reading D.C. Waldorf's book, "The Art of Flintknapping". The short Proof is seen in very BB hole you have ever seen in glass. The 100 degree cone of the hole opposite of the point of impact is the same degree of fracture that is characteristic of all rocks made from silica. Knapping a flint pre-form uses those angles to make your gunflints.
If you want to knapp your flint, to resharpen the edge, simply empty the gun, cradle it in your Left arm( for RH actions) lift the frizzen up with the inside of your left thumb, next to the nail, making sure to keep the end of your thumb BACK away from the face of the frizzen. Cock the hammer with your right thumb, and lower it down to help you align the bottom 1/4" of the Face of the frizzen( the heel) with the edge of your flint. Now hold the frizzen in that position, cock the hammer back to full cock, and pull the trigger. The flint will strike the heel of the frizzen at this much steeper angle, knocking off a bit of the edge across THE ENTIRE WIDTH OF THE FRIZZEN, leaving you with a new, sharp edge, all across the width of the frizzen. It will knock off a long spawl from the bottom side of the edge of you flint, leaving the flint sharp, with a much more narrow bevel to the edge. You are back in action.