MongoClient supports the connectTimeoutMS, parameter:
http://api.mongodb.org/python/current/api/pymongo/mongo_client.html

connectTimeoutMS: (integer or None) How long (in milliseconds) a connection can take to be opened before timing out. Defaults to 20000.

However with the latest Topology bits connections spend server_selection_timeout waiting for the initial connection (this is set to 30s by default):

select_servers [topology.py:106]	
select_server [topology.py:120]	
_get_socket [mongo_client.py:651]	
__enter__ [contextlib.py:17]	
_socket_for_reads [mongo_client.py:687]	
__enter__ [contextlib.py:17]	
command [database.py:451]	
server_info [mongo_client.py:886]	

It would be good if connectTimeoutMS - which is the only parameter defined as API - influenced server_selection_timeout.

Looking at the code it is possible to override server_selection_timeout by passing: serverselectiontimeoutms, but I can't find this documented.