When presented with a list of criteria, users/lookup will respond with objects that have a match. If an object has no match, it just won’t be returned. So if you provide 100 user names, but only 50 of them match Twitter accounts, you’ll get 50 records back and no indication as to the other 50 you were looking for.
By time I mean:
The REST API is rate limited. Each API method has a different rate limit. Using OAuth 1.0A, you can use users/lookup 180 times per 15 minutes. With 100 lookups per call, that allows you 18,000 lookups per 15 minutes. Once you’ve looked up 18,000, you’ll have to wait until a 15 minute window is complete before resuming.
All the information around rate limiting is communicated via HTTP headers. You can code your application to listen to these HTTP headers and throttle requests appropriately.
When performing bulk user lookup like this, it’s best to be especially polite with the API – respond to error codes and rate limiting conditions… if the API tells you it’s having trouble processing your requests, exponentially back off.