It really depends. If you both have already had sex, then talking about things like that is really something that you both should be open to. However, I wouldn't worry about it too much. Most women are okay with most of the basics like restraints or roleplay. That's really becoming really accepted in our culture. I recommend that you start with something small - after you both have been dating and it's going to be a serious thing and you know that, I recommend asking her if she'd be willing to try something like velcro handcuffs or some silk scarves for restraints. Starting out small and putting more advanced things into it slowly will make her much less likely to run off. As for how long to wait - I'd say until it's obvious that the two of you want to date long-term and have a stake in the relationship.