Of course, the really big questions take time, and the Virtual Reference Desk would tell you that. Each service must state the percentage of queries it actually answers. It also has to give the typical response time. Most of the services rely on volunteers. So you get very smart people, but they don't sit in front of their computers waiting to answer.
Responses usually take a day or longer.
And because you're often reaching educators, you may get a response that's more educational than it is an outright answer. Still, the results can berewarding. Lucia Cataldo, the head librarian at Oswego High School, recalls a 10th grade biology project. Students had to choose an environmental career, research it, then interview a professional to see how their research compared to the actual experience. They reached these experts via the Internet.