When manual user is created from webservices, if timestart and timeend values are not provided by the caller, these are set to 0 when the enrolment record is created. The enrolperiod for the manual enrolment instance is ignored and thus the user is enrolled indefinitely.
You can see this in enrol/manual/externallib.php. I think if these are not passed in, timestart should be set to now (time()) and timeend set to now+enrolperiod if that is set, which is how these are set if creating the manual enrolment through the web page. Any reason why these are both set to 0?