To call a web service activity repeatedly in Cast Iron you can use either ‘While Loop’ or ‘For Each’ activity. The attached project illustrates retrieving opportunities data from Siebel CRM On Demand (SOD) and puts this data as a text file on an FTP server. In the project I invoked the ‘OpportunityQueryPage’ web service call in a ‘While Loop’ by specifying the 'pagesize' and the 'pagenumber' as input. The ‘While Loop’ repeats until the value of 'LastPage' returned by web service call is 'true'. The retrieved data is serialized and appended to the text file on the FTP sever in each iteration.
I believe the limitation of ‘SOD web service requires that successive web service calls for a given session be spanned by a period of time no less than 50 MS’ may be specific to the server you are using. I could not find any such kind of limitation on the server I tested with the attached project and also in the SOD documentation.