I've tried cut flints, they don't seem to work as well for me in most cases as English knapped flints. When it stops sparking well, you just look for the spot on the edge that is dull, then tap/flake that one spot off. Sometimes it might be more than one dull spot....but rarely does the whole edge need knapping to restore the sparks.
I usually get 15+ shots from my .50 rifle before I might need to sharpen the edge by flaking off a dull spot. I usually get 90+ shots from each flint in this gun. Adjusted in the jaws a couple times as it gets shorter.
I get 30+ shots from my Charleville musket before anything might need sharpening. It's a bruiser of a lock and seems to spark regardless. The current flint has over 80 shots on it and plenty of life left. The flint it replaced had 140+ shots on it....and I only replaced it because it had broken into two pieces and by that time was only about 3/8" long! Still sparked up a storm though. I have a photo of that flint around here somewhere...down to the nubs it was! That musket is my favorite "plinking gun", because it just shoots and shoots and shoots...with no muss no fuss.
Jack