The Login operation is no longer required or recommended; instead, to access PPM Pro, WS API users should be given an authentication token generated by the PPM Pro administrator. See Authentication Token.
You can start a web service call with a Login operation, which returns a session ID that is required for all subsequent method calls. A session is valid for 10 minutes, and resets with every call made within the 10 minute period. So if you get a session ID and make a few calls in a row you won't need another login call. But if you wait more than 10 minutes, you'll need to issue Login to get another session ID and use that in a subsequent call.
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://services">
<ns:return>F0C6EDD44B270B1F3DD0F1492A2A1585</ns:return> <!--Returns SessionId-->