When is the correct time Tzeit Hakochavim? I have always used the Chabad website which uses 3 Medium Stars, approx. 24 minutes as degrees after shkiah as the default, but I know there are other views. For example, some wait 72 minutes after shkiah.
Technically we can say Kriyas Shema when there are three medium stars. The Shulchan Aruch (O:CH 235-1) however says that since we don’t really know which stars are medium, and which are large, (which can be seen even during the daytime), therefore we wait until we see three small stars.
The exact amount of time after sunset is not clear, and it depends on your location, and how fast it gets dark in the part of the world. For example, it gets darker quicker in Eretz Yisroel, than it does in the States and in England. If you are located very north, then 24 minutes seems to be very quick. However, in Eretz Yisroel, there are many that will say Kriyas Shema at that time, however others will wait until 30-40 minutes after sunset.