Contractor Finder Fee

So, I got an offer for a contract job setting up a File Server and Exchange Server for a small company.

The person who pointed the company my way talked to me about a finders fee for pointing them my way. Does anyone have any experience in this?

What is reasonable and customary?
How long does the person get the fee if I continue doing work with the company?

I think this is new to both me and the other person.