If you are interested in the job, you can go ahead and answer client's questions about how you would complete the job. If you need more information to answer the question, you can ask the client to clarify.
I agree with Sheressa! The only time I ever give a client an hour estimate is if I think I can get a job done in an hour. Usually, though, I use days to measure time. For example, a client asked me to track down 250 leads recently, and I told him it'd take a couple of days (if I didn't have any other work those days and could dedicate an entire work day just for this job). Besides that, I can't really give estimates, especially when working in a new program.
Agreed with both of you. I think the best solution to this is to convince the client to pay you per lead. I usually do that and it works best. I think hourly jobs don't work best when it comes to lead generation.