How full can my sump pit get before I should worry?
It should definately come on before the water gets to that level. Is the float the kind that has its cord clamped to a pipe and it swings up and down? If its that kind, make sure that its not hung up on anything. You may need to manually run the pump to empty the water and troubleshoot the float. Lift it and see if the pump comes on. Does the float have a separate cord that plugs into the outlet and then the pump plugs into the back of the floats plug?

I agree that it should pump more than a half inch per minute. I would think it would be normal to empty the tank in around 20-30 seconds.