FWB...are there any rules? Need help/suggestions please...?

Monday I hooked up with a guy I've been talking to, we had and it was a really good experience. When I was leaving he said he wanted me to come back again sometime and I said I would like that. Then his roommate came in and I left. This is strictly a physical relationship which at this point of my life I'm okay with. But I've never been in a purely physical relationship before and am not sure what the rules are. Was it okay for me to send him an email the next afternoon, telling him how good it was and that I think we should do it again? It was nothing too dramatic, kinda light and humorous. How long should I wait to try to hook up again? I don't want to seem too eager for fear that he thinks I'm getting romantic feelings for him and want a relationship. Is it even okay for me to tell him when I want to get together again or should I wait for him to make the next move towards that?