Pulling data from external system via web service

Hello.

I have a scenario where for about a 100 users i need to call a web service to get data from our own on demand solution into salesforce for a salesforce integration.

The web service takes the user information as an input

- Questions are:

1. Is the best way to get this done is to sit in a loop in apex can call the web service a 100 times or is there a better way?

2. Is there a time out concept i.e if all 100 are not done, then how does the application recognize that.. and even if we track the number of users still left to loop through, does user manually have to make this call again or can we do this programmatically.

3. Or another option is to just send one quick outbound message to our app for all the data needed, and then the web service pushes all the data into salesforce.

Any pointers would be greatly appreciated.

Thanks