When your dating someone how long do you wait to have the how many past sexual relationships have you had talk?

Definitely not right away, and it depends what you want to know and why. If all you want to know is whether they're clean of STD's, you can just if they've been tested recently before you consider getting sexually involved with them. But if you want to know about level of experience and attitudes towards sex, that might be a conversation reserved for a time when you really know each other.