In my case I used a digital input. So I check the digital input during that 4 ... 1200 ms.
In your case, you use the analog value. So 1000 ms is (far) too long I think. But that all depends on the led and the amount of light.
Since this is done with the capacitance of the led and the circuit is high impedance, every little electric noise will influence the reading. Your hands will probably introduce noise from the mains 50Hz or 60Hz into the circuit. The longer the delay, the more this is a problem.