OneWay operations don't wait for a Reply Message. It just writes data to the network connection and returns. So, the only "wait time" would be the time required to write the message to the network.
Be aware though that WCF can still block the client (Clients Blocking with One-Way Operations):
this means that any problem writing the data to the transport prevents the client from returning. Depending upon the problem, the result could be an exception or a delay in sending messages to the service.
Edit: Regarding timeout, they are set on the binding. If your operation can't perform his "Send Message", it can still timeout.