Revert "fix: Update token refresh threshold from 20 seconds to 5 minu…

…tes (#1146)" (#1153)

This reverts commit 261a561.

google/auth/_helpers.py

@@ -23,7 +23,11 @@
 from six.moves import urllib
 
 
-REFRESH_THRESHOLD = datetime.timedelta(seconds=300)
+# Token server doesn't provide a new a token when doing refresh unless the
+# token is expiring within 30 seconds, so refresh threshold should not be
+# more than 30 seconds. Otherwise auth lib will send tons of refresh requests
+# until 30 seconds before the expiration, and cause a spike of CPU usage.
+REFRESH_THRESHOLD = datetime.timedelta(seconds=20)
 
 
 def copy_docstring(source_class):