How often does Python switch threads?

Since Python 3.2 the interpreter’s thread switch time interval can be adjusted using sys.setswitchinterval: https://docs.python.org/3.6/library/sys.html#sys.setswitchinterval

In earlier versions the interpreter checks for periodic things such as thread switches and signal handlers by default every 100 virtual instructions. This can be modified by sys.setcheckinterval: https://docs.python.org/2/library/sys.html#sys.setcheckinterval

By default, Python 2 will switch threads every 100 instructions. This can be adjusted with sys.setcheckinterval which is documented here: https://docs.python.org/2/library/sys.html#sys.setcheckinterval

I found additional information on pages 10, 11, and 12 of this presentation: http://www.dabeaz.com/python/UnderstandingGIL.pdf

(These answers come from the comments of the question, but since nobody has written them down in an answer I will do it myself. It has been 6+ months since the comments were made.)

UPDATE:

Since Python 3.2, sys.setcheckinterval was deprecated. CPython has another approach to improve the GIL. Instead of release the GIL after 100 virtual instruction, they changed to switch by time interval instead.

By default, the GIL will be released after 5 milliseconds (5000 microseconds) so that other thread can have a change to acquire the GIL. And you can change this default value by using sys.setswitchinterval