Another idea. The concern is that the gas valve opens just seconds
after the pilot flame lights, you think it should take longer to detect
that the pilot is lit. So, rig it up so you have your meter on the
wires to the gas valve. Then turn off the gas and fire the furnace up.
If the flame sensor is working, with no gas, no pilot, the gas valve
should not get voltage to open.

I know that main gas is coming on (when it's doing this) irrespective of the status of the thermostatic switch that gets heated by the pilot. I know this because of an LED I have on the hot side output of that switch AND another I placed on the main gas valve input wire on the gas valve. (I believe these are just opposite ends of same wire.)

With gas shut off as you suggest there would be no way to see that the main gas valve is opening. My indicator LEDs already tell me this is happening without the pilot heat switch being hot.

I just watched it and while I didn't run a stop-watch I'd say it normally takes about 45 seconds for the pilot heat switch to switch main gas on. Naturally this is going to vary with how long it's been since the burner last cycled and how much that part has cooled.