What is a typical response time for a photodiode/phototransistor?
If you cannot test as arielCo suggests, (or even if you can) you should look at the specs for your device. If you search digikey for optical sensors - transistor output the candidates will have response times listed from 20 nsec to 1 msec. If you search for optical sensors - photodiodes the listed response times range from 30 psec to 2 msec.

Turn off when you block the light will be slower than turn on, and won't really start until the light is completely blocked.