Rule of thumb is that you should take the shot within the second after you line up your sights and acquire a sharp front sight picture.
That said, I shoot 50~150 rounds per hour, depending on my performance that day. On some days I feel like I can shoot from my hip and beat the living you know what out of the bullseye. On other days I am not sure if I can even hit the paper.
For days where I doubt I can even hit the paper, I load some snap caps and dry fire at the target. It rarely gets better until my next visit, but at least I don't waste ammunition for that day.