I did some experimentation with this and I found that the PI's GPIO pins wern't accurate enough to detect the pulses from an LDR, so I ended up using a dedicated microcontroller to connect to the sensor then reading that into node-red on the pi over serial.
I have the same question too. PI's GPIO is not accurate in timing compared to FPGA, but it can still reach us (microsecond) level accuracy, which should be more than good enough to detect most pulses. But I don't know about timing accuracy of PI GPIO in Node-RED.
Unless he is checking the level of the light, in which case he needs an ADC.
...or if you are not intending to use a database, you can save & increment the values in context (create a running total) see Working with context : Node-RED
As Colin has said, it depends what you intend to do with the data.
So all you need is the rate at which messages are coming in. If you search the node red flows site for 'speed' or 'rate' you will find a number of possible nodes. For example node-red-contrib-msg-speed (node) - Node-RED might do what you want.